I think this is misconception many folks have, and it is quite pervasive throughout Europe (i.e. it is not limited to the UK). I think there are several reasons for that, but I feel it is first necessary to contextualize the term racism a bit. Many folks see racism as the expression of racist attitudes or sentiments by individuals which range from stereotyping to harboring certain ideals of racial superiority (or inferiority). That in itself is not a issue in isolation, as that would be individuals being arseholes and you will find them in each society. What is different is mostly what is considered permissible (to state openly) , which in turn are obviously heavily influenced by respective histories. There is an obvious difference in black-white relationship due to the historic suppression of black African American communities up until very recently, for example. There are also different entanglements between certain races, social attitudes, influence of wealth and class and so on. It is relevant to state that most of our Western modern thinking about race and associated stereotypes are heavily influenced by enlightenment theories on human races, which, in turn, were strong affected by colonialist attitudes. As such you will find in quite a few Western countries, presumably also in the UK (where my knowledge is at best second hand) but certainly in Germany the stereotype that e.g. black folks are more physical and aggressive, for example. As such while there are interesting overlaps, the expression and permissibility of racist attitudes between countries is nuanced and it is easier to talk about the difference in form and impact rather than level.
One cannot really state that racist attitudes are not as pervasive as they are in the US. Europe as a whole has many ethnocentric tendencies (which is far less nuanced in the US), which we see very prominent with the rise of popularism throughout Europe and movement such as Brexit have been heavily influenced by explicit and implicit prejudice. While not all of them are along racial lines, it is undeniable that these are strongly correlated. In Germany many folks make a distinction between ethno-Germans (sometimes semi-jokingly called Bio-Deutsche) and those with a migration background, but rather obviously the latter are singled especially if they are non-white.
However, that is not the whole story and perhaps not even the important one. As mentioned racism as a phenomenon on the individual level is not a huge issue per se, but it begins to become an issue if they result in systemic effects. This is often why folks distinguish between racist attitudes within minority and majority groups as the latter can lead to issues that are more commonly discussed academically. These issues include racial discrimination and racial inequality. While it is easy to conflate these terms there are very different mechanisms at play. For example racist attitudes can be foundational in the creation of either racial discrimination or inequality, it does need to persist in order to continue. Often things like implicit bias rather explicit belief in racial superiority are important drivers or even just historic decisions that have not been questioned. Even something as simple as not addressing issues that are not deemed important by the majority but have significant impact on minorities can create racial inequality.
In that light many parts of Europe do have similar patterns as the US. Some of them are borne by the fact that minorities traditionally (but less so in recent immigrants) have been working in low-skill jobs. But at the same time multiple studies have found discriminatory practices where certain minorities with same CVs are evaluated worse, for example or are less likely employed, have less social mobility than their equally poor majority counterparts and so on. A big difference is that since there is not such an overt historic conflict, it is rarely discussed as openly as currently in the US. There has always been the demand that minorities should assimilate and thereby become invisible as such, which obviously does not work well with visible minorities. What is different is potentially (but I am not well versed in UK politics) is that in the US there is a more concerted effort in suppressing the rights of African Americans. Such voter suppression strategies are, to my knowledge, not present in (most) European systems. However, historically (perhaps less so in the UK due to their empire) minorities in Europe often had little political engagement as a whole. Many, even those in the second or third generation were still seen as foreigners or immigrants rather than full citizens. But in recent years I have seen an attitude change (but, as noted, there are also strong countermovements).
So here we have a needlessly long answer which could presumably be summarized that a) on needs to define more clearly what one means with racism and b) whatever it is, it is difficult to quantify except some of its effects and c) racism and its effects are different between countries but I am not certain whether I would subscribe that the US is more (or less) racist (again, which measures?) than the UK.