Options
1. Do it now and have a BSS cert dated from the test date
2. Do it within the 2 months before you go - which may not be possible
3. Ask CRT and they will -
A). Say it’s ok to be a bit late, but probably they won’t as the rules are clear
B). Say it is definitely not ok. In which case it’s either option 1) or ignore CRT instruction. Not wise.
4. Say nothing, do it as soon as you return. If CRT notice which I doubt, say ‘very sorry, I tested as soon as I realised I was late’.
I know which I would do as it’s always better to say sorry than try to explain why you disobeyed an instruction. But if you don’t like being a bit naughty, or worrying about insurance, upsetting CRT etc then it’s option 1.