Goodness, that's an awful lot of money - The Big Plant Nursery is the place to look at, its in Berkshire, but they do online as well - although for the size of Trachycarpus you want (240mm plus, £400 each) you'd need to ring to discuss delivery. Just google Big Plant Nursery and it should come up. And I assume you're aware that mature, large plants don't always take well to being transplanted, unlike their younger siblings.

Architectural plants http://www.architecturalplants.com/

The RHS website lists 40+ suppliers up and down the country, most of those do mail order, and there are nurseries and GCs that don't list themselves with the RHS, so it would seem that you won't be too far from a supplier - whether any one place would have the size and quantity you are looking for is another matter (best kept between you and your bank manager I would suggest!)

urban jungle Norwich Norfolk have a great web site and some amazing plants .

Mypalmshop will be cheaper than the above given the Euro to GBP exchange http://www.mypalmshop.com/Trachycarpus%20Fortunei.

Urbanjungle is good and they have an up to 50% off sale in very early January (Trachies tend not to be that low in price).

Nigel at Hardypalms in Bristol grows them from seed in the UK, so they are more hardy:

http://www.hardy-palms.co.uk/category.asp?section=16

You should also look at ebay - here is a 3m tall Trachie for free if you dig it up:

http://www.ebay.co.uk/itm/Tall-Palm-Tree-Plant-for-Free-3-60M-tall-/271787019617

also try gumtree

Botanic's EBay answer might save you more than importing and you won't have import issues to contend with - importing plants isn't like bringing in a couple of boxes of wine! And I doubt if you could import such large specimens significantly cheaper than the retail price when you've. Added import duties, shipping, road freight etc onto the purchase price.

I agree with urbanite .