Beg pardon. Lots of people think there's a rule but it isn't straightforward:
So if you are a genuine refugee your claim will not be rejected on the
basis that you did not claim asylum in the first country you came to,
but you may be passed from one country to another before your claim is
determined and the effect of the 2004 act is that it is extremely
difficult for an asylum seeker to challenge removal from the UK to a
country deemed to be "safe" – particularly if the removal is pursuant
to Dublin II.