Along with so-called 'critical race theorists' I understand racism not as individual prejudices and general bigotry, but as the process whereby societies hierarchically organise 'races' and distribute resources unequally according to this hierarchy. Across Europe and the Americas whiteness (or 'white') is at the top of this racial hierarchy. What are the benefits of being at the 'top' of this hierarchy? Not only do you receive 'privileges' or access to certain resources that others don't have (such as in employment, housing, education, health care and so on), but you also have the ability to produce the 'ruling' ideology(/ideologies) which reproduce the racial hierarchy - in other words they maintain the 'racial status quo'.
“During the last three recessions, black unemployment in the UK was around 19% higher than in the US. But that doesn’t automatically mean that the UK is ‘more racist’ than the US.”
This definition, or broad understanding, of racism makes it hard to have rigorous comparisons of racism. This is partly because there is no uncontentious way in which to 'measure' the extent to which different societies are differently organised according to race. Take income inequality for instance, some countries such as France don't even collect data on citizens' 'race', so any comparison here already meets a dead-end.
Then there's the problem with the fact that racial hierarchies, and the mechanisms that reproduce them, are multifaceted and thus vary across the globe. In the UK, during the last three recessions, black unemployment was around 19% higher than black unemployment in the US in the same time periods. Yet, this doesn't suggest the UK is 'more racist' than the US - despite the fact that police brutality is a reality Black-Brits face too, it's matter of fact that it's much more intense and murderous in the US.
“In Mexico it’s claimed that there is no racism because the population is ‘mixed’ between European and indigenous. But racism against the African diaspora in Mexico is still rife.”
There has thus been a move in the social sciences (in which I place myself), which stresses the need for 'relational' rather than 'comparative' studies of racism. The 'relational' approach stresses firstly how racism is a global phenomenon. It reaches into those states even where there appears to be 'official' statements of non-racialism (for instance, in Mexico there's the claim that there is no racism because the population is 'mixed' between European and indigenous - however, racism against the African diaspora in Mexico is still rife). Given that it is global, or at least incredibly well spread across the globe, a relational approach to racism analyses how nation-states 'borrow' or 'take and make adjustments to' mechanisms that other states use to reproduce their racial hierarchies. In other words, there are stark similarities (although not complete replicas) in the ways in which nation states reproduce their racial hierarchies.
Some examples to highlight this relational approach. Firstly, we might want to point out to the fact that across the globe similar 'meanings' are associated with blackness. In the US, South America, Europe, India and even in 'black majority' countries such as South Africa, blackness is associated with violence (especially black men), and black women are portrayed as sexually promiscuous. This is often used to justify the state brutality toward, and spatial exclusion (in housing), of black populations across the globe.
In terms of a more contemporary example, think about the rise and normalisation of far-right, anti-immigrant, nationalist discourse across the globe. Take Trump in the US, Brexit and UKIP in the UK, Front National in France, and the Sweden Democrats - they all mobilise extremely similar radicalised ideologies. These racial ideologies feed both on a white nationalism (taking 'our' country back), and a framing of the dark-skinned immigrant as a national threat (framing them as rapists, terrorists, or simply a drain on public resources).
“There is a rise and normalisation of far-right, anti-immigrant, nationalist discourse across the globe. It feeds both on a white nationalism and a framing of the dark-skinned immigrant as a national threat.”
So, long story short, I don't have an answer to whether Britain is more or less racist than other states. I would pay attention to the fact that through colonialism and Empire, Britain was one of the leading 'exporters' of racial meanings (e.g. the idea that the black body was cursed by God, that blackness is sinful etc.), and one of the key 'powers' responsible for the current global domination of blackness.
That being said, British colonialism (although many of its discourses are still alive and well) as a political project is no longer the central way in which the racial hierarchy is reproduced. The mechanisms that reproduce racism in Britain actually appear to be extremely similar to those across the US and Europe - housing segregation (and not just self-segregation - also 'white flight' away from areas where that are nonwhites thus leading to some areas across the UK being 90/95% white just like the US); this segregation leads to unequal access to resources such as decent education and healthcare; a mainstream media and political party(/ies) that reproduce negative/stereotypical/racist portrayals of nonwhites; and police violence justified on the basis of the 'ruling' ideologies of race (such as Asian men as suspect terrorists, blackness as an aggressive 'bio-cultural' characteristic and so on).
There probably are ways in which we can measure difference intensities of racism. However, I think that it is more fruitful to adopt a relational approach when we can see how racial inequality across states often has its basis in extremely similar circumstances (although, naturally, there will be nuances). These extremely similar circumstances are not coincidental, but represent how 'racism' (and 'race' more generally) has a global hold.