This raises so many questions for me - must it be honeywell like the
junction box it connects to?, must it be a particular type?, what size
does it need to be?, will the wiring requirements of a different valve
exactly match those of the old one?, where is the draincock?, do I have
to drain the whole system?, etc. etc.

If you are replacing the entire valve, the main compatibility issue is that
the connections are at exactly the same place and the thread (either fine or
medium) matches the original nuts. If so, no pipework modification or olive
removal is required.

The other main issue is making sure it is the same type of valve (i.e. mid
position, diverter etc).

However, you might as well replace with another Honeywell. They are well
regarded and not that much extra in cost.

Christian.