Hi Adrian. I have only just spotted this
thread: my apologies if the issue is now academic.
I'm in the UK too and 4 or 5 years ago when my
collection was much bigger than it is now I managed to get it insured along with my general household contents through one of the usual general insurers (Liverpool
Victoria). I simply paid an additional amount (it wasn't very much, £50 or so if I remember correctly) to increase the section of the cover for named valuables - the
collection was regarded as a single valuable for this purpose (I did not need to list individual coins). That increased this
part of the cover from the normal £5000 to £40,000.
I dispensed with the cover a couple of years ago when I sold all my
Romans, but they may well
still offer it.