Each NHS funded cycle will usually consist of a fresh transfer plus 1/2/3 or unlimited transfer of frozen embryos this isn't always true though, each CCG sets its own funding rules. In mine they funded two fresh rounds, and one FET from each fresh round if that makes sense.
OP, you need to find out your CCGs funding policy as this will inform your decision.
Additionally you need to consider how you'll feel if you defer for your career and things take far longer than expected. We started IVF when I was 28, no female factors whatsoever. Straightforward icsi should have resolved our problems easily. However I reacted badly to IVF, had many complications and it took four rounds to get my son. We've since done a further four trying for a sibling. Time is of the essence with IVF, if you're unlucky enough to end up with a journey like ours. Luckily, because we started young we still have time to continue to pursue the family we want.
On the other hand you may get lucky and it works first time...but there is no way to tell.
I understand the dilemma, I really do. I am the main earner and I love my job. I was in a fab job that paid well and had great maternity benefits but was made redundant the day after I found out our fourth IVF worked-not related at all, but proof that you can think you're in the best position and things can still go south!
The point I'm making, is work out what's most important and prioritize that. If you want a family, don't delay, especially as you know you need IVF.