100mW (20dBi) is the legal limit for 2.4GHz band. Anything above that could land you in hot water if you got caught. These units have been available in the US (where they are legal I understand) for years.
When I was involved in this kind of thing - it's been a few years now - we used a company called Hyperlink Technologies in the US.
www.hyperlinktech.com. There may be competitors in the UK now, but at the time this was the most cost effective way of getting hold of aerials, feeders, etc
BTW, I've got a couple of aerials still in their original boxes if you are thinking of doing long range links. By all means drop me a PM if you are looking to buy anything of the sort.
Chris