I used to work in a shoe shop for a long ol' time and it took me ages to realise why there's so much inconsistency between european and UK sizes.
Basically, the difference between 2 European sizes is a lot smaller than 2 UK sizes (e.g. the difference between 38 & 39 is smaller than 5 & 6) so they just don't line up - and most brands just round to the closest.
Unfortunately, a european 40 is pretty much dead on a UK 6.5 which is what throws everything out - it can be pot luck as to which fits better.
So, after being no help at all, I'd say try the shoes on before buying, and if you're buying online, factor in time to make an exchange if you have an event coming up!