When the UK CAA was a part of EASA or JAA, if a pilot had several thousand hours in command of a +30 tonne aircraft in primarily international operations, they only had to do a couple of exams.
Now... nope. UK CAA left EASA (silly decision IMHO) so unless you held an EASA ATPL issued by the CAA then you'll have to comply with this
validation that lasts for a year.. (I am taking advantage of the year's grace to get my UK ATPL "back").
There's some information
here but it's a bit thin.