The European casinos can only accept UK players if they have a UKGC license. If the European casino has a UKGC licence then the players from the UK will have bonus buys disabled.
Sometimes, as is probably the case with Drago on Conquestador, a casino forgets to disable the bonus buy or, after an update to the operators platform, switches will get flipped.. There was a Nolimit game at XXXXX where the bonus buy was enabled for me, a UK player, I told XXXXX that it looks like they forgot to disable the bonus buy to the UK market, they contacted the relevant team and said "yes, it looks like it" "thankyou" and then they disabled the bonusbuy for the UK region.
The only European casinos that will accept you, a UK player, when they do not have a UKGC licence are very likely to be scams.