The area will be inflamed for a while. Additionally, the rectal wall in contact with the prostate will have received pretty much the full radiation dose (unless you had a rectal spacer installed). You also have the blood vessels supplying the penis just behind there, and these are liable to late-onset damage from the radiation which will impact erections, so you might want to give then time to heal too, although I've no idea if this would increase the chance of late-onset damage.
I think I would suggest holding fire for some time, and using plenty of lube when you do, because rectal wall at that point may be damaged and more fragile (radiation proctitis).
Also, refrain from this for many days before any PSA test, just like you would for a DRE.
The question is absolutely fine to ask, but I think you'll struggle to find the expertise to answer it. You should have been given a number to call to ask questions, and you might consider asking a consultant via that. One thing you might look at is the PCUK Guide for Gay and Bisexual Men which although not your case, does cover anal sex after radiotherapy treatment which is closely relevant.