If new, Edinburgh Bicycle (http://www.edinburghbicycle.com) Country Traveller tourers get good reviews, and come pretty much built to go. they are cheaper and better spec'd than the larger brands (specialized, cannondale, etc), for the money. their '07 bike currently has £100 off, and is now £299. You'd have to check sizes. I've ridden one and it was good comfort-wise, but I chose to go off-road so an MTB.

Ortleib dry bags are about £70, and the front bar bag is about £50.

If second hand (and I've just been through this) you need to be un-fussy about the bike. But I've since found that some bike shops carry old/sh road-frames for sale cheap. If buying second-hand you're probably looking at an additional £30 for a pannier rack, chain (£15), brake blocks (£10), tyres (£20), etc, etc. it adds up quickly. I gave up on my second-hand search as I couldn't find something my size (6'), and you really have to be prepared to strip and rebuild with the cost of cables and seized parts.

at that price though it's not going to be particularly light. I have a road frame (raleigh R100) which is light-ish (steel), but can't take touring tyres and a rear rack is a pain to fit and isn't all together stable.

I have an EB Cuillin Sport (£280), plus panniers (£70), plus rack (£30), plus bar bag (£50) and tyres (£20). It's heavy, but have done a few 50 mile rides and it's fine. doing three weeks road riding in france from friday to try it out.

the Kona Smoke has also been suggested here.