Pat,
It sounds like you're after the bottom metal that Remington fit to their rifles that have a detachable magazine.
You might struggle to get a replacement in the UK as most people swap the very common sprung floorplate for an aftermarket detachable mag system. I'm not sure how many people would be likely to swap a Remy detachable mag system for an after market one, hence freeing up a spare set of Remy detachable mag bottom metal for you.
If a replacement Remy part doesn't turn up the Anglo Custom one is good, although as you say, you'd need an AI magazine for that, which are pricey.
Can't think of another way to do it, except perhaps getting someone from the states to send you the part. Might also be worth a look on the Midway site. They do alot of smithing spares.
I looked into this a few times as I was looking at Remys but wanted a detachable mag system. The rifle I ended up with had the AICS system, which was ace. Unfortunately it upset Mrs DB and I had to sell it.
Best of luck with it.
Cheers.
Bob