Thank you everyone for the very useful information, much appreciated.

I don't have premium Amex, just the normal one. But yes, the 241 would expire. My Amex "year" starts around 19th Feb and I don't normally have a problem reaching the required spend (£20k) within a few months as I put all my business expenses on it. So worst case it may be April or May before I have a new 241. By then I should also have reached 2500 TP (1400 already booked, my year resets on 6th August), so in theory I will have a 241, GUF2 and a joker(s). BUT will I then have left things too late to book something? Am I correct in saying that a GUF2 used via a TA still gives me more options in terms of availability?

Is this the sort of thing I can call the GGL line about and seek their views? I've never actually called them before other than to change one flight.

Also, is there any way of swapping to a premium Amex to benefit from the lower mileage requirement? I suspect not, as I recall you can't qualify unless you've had 6 months between cards.