sliat_1981:
Give me a break, are the British publishers so up themselves that everything had to be British?
From a modern perspective, I agree it seems surprising that a British publisher would feel its young readers were so parochial that they'd be fazed by a reference to St Peter's in Rome and need it to be changed to a familiar London church. Similarly, the whole decision to Anglicise the locations of Tintin's home, with that
Marlinshire, England address in The Secret of the Unicorn, seems oddly little-Englandish from today's perspective. These days, children's books being translated into English by UK publishers always seem to maintain the foreign locations and character nationalities of the original books, and quite rightly so. And if the Tintin books were being translated into English for the first time today, I'm sure that would be the case with them too.
However, you have to remember that Methuen were first publishing these books in the late 1950s, attempting to launch them into a market where many adults (parents, teachers, librarians, etc) regarded all comics as worthless pulp, and where many comic-loving children would have found the idea of a Belgian hero unfamiliar. Whilst it's quite possible that Methuen were underestimating children even back then and that maintaining more of the Belgian flavour might have gone down fine, the translators were doing everything they could to maximise the chances of what was seen as a risky venture. (Previous attempts to launch the books in English by the Belgian publisher Casterman had failed.)
Maybe not all the Anglicisations were necessary, but I think the general policy of the translators, Leslie Lonsdale-Cooper and Michael Turner, of prioritising a natural-sounding English text over a strict word-for-word translation was a big factor in the books' success in the UK. You only have to read some of the recent Blake and Mortimer English translations, or some of Michael Farr's translations of Phillipe Goddin's books about Hergé, to see how clunky a word-for word translation of French syntax can be.
In any case, Hergé apparently wholeheartedly approved of the English translators' approach.
In this particular instance, I guess that the translators felt that having Haddock saying "according to your calculations we're now standing inside Westminster Abbey" better maintained Hergé's intention of an understated and instantly gettable gag than a more faithful translation would have done, in that Westminster Abbey would be as instantly familiar and obvious to most British kids as St Peter's would be to most Catholic Belgian or French kids.