Gavin-
The Rimmers site is a little painful, but you can get the price online. First, download their pdf TR3 catalog. Post 60K PVC wiring loom is part number 207162PVC. Then plug that number into their price search, and you'll get: 104.68 GBP or $157 excluding vat. 20% off gets about $30 knocked off, I think shipping will be about $40, which gets you to about $170-ish delivered.
A word of caution though. I haven't fitted the loom, so make sure it is what you want, and if I made a mistake, don't repeat it. For example, I'm pretty sure looms for the TR3As are not left/right handed like the later cars, but if they are I just bought the wrong loom..
If you can I'd advise calling in the order - they are very fast and you'll get around any issues that might crop up with online ordering if it is more set up for the UK.
Randy