All the Showa releases are available on Region-2 (UK compatiable) DVD from Japan, complete with English subtitles (bar GAMERA: SUPER MONSTER, for some bizarre reason). The two box sets ("1965-1968", and "1969-1980") carry a hefty price tag although make a wonderful collectors piece, however at the same time Showa Gamera just isn't for everyone. Perhaps you should consider picking up one or two individual discs first:
http://www.cdjapan.co.jp/search.html?ty ... ord=gamera - I'd suggest some of the earlier films such as "GAMERA" or "GAMERA VS. GYAOS" personally.
As Keith said, Kaneko's 90's trilogy is/will be available in the UK. "GAMERA: GUARDIAN OF THE UNIVERSE" was dubbed with those stereotypical "BBC voices" and a load of UK techno music replaced Kow Otani's score. It was released by Manga on VHS quite a few years ago, and also had a short theatrical double-bill with "THE LOST WORLD: JURASSIC PARK" in the UK back in 1997. I'd really suggest picking up ADV's disc from the States instead, as it's both DVD and un-butchered (sans the title card). "GAMERA 2: ADVENT OF LEGION" was released on DVD back in January in the UK. I picked it up, and it's very good, I'd highly recommend it. "GAMERA 3: REVENGE OF IRIS" is due for release this time next week.
Unfortunately, Manga still hold the copyright to "GAMERA: GUARDIAN OF THE UNIVERSE" in the UK, so we're stuck with their badly edited version which they ended up giving away on DVD free with a PlayStation 2 magazine "PLAYNATION" a while back. You can usually pick it up off eBay if you're interested - it holds an odd nostalgic charm for myself being the first version of the film I saw, but it's really a terrible release in all reality.