A normal road bike - what some people call a racer - drop handlebars etc.
£500 used to be the entry level for a decent bike - something that is 99% as good as a bike costing much more - things have changed a bit in the last year or two as prices have gone up but you should still be able to get something good for that if you shop around. Shops sometimes start selling their current bikes off around Sept time ready for the 2011 stock so that's something to think about - right now at the height of Summer and the Tour de France just finishing there are likely to be fewer deals.
If you want to compete in triathlon you need a helmet- whether you wear one for training/commuting is up to you. Say mabe £20 upwards - most probably pay more like £50. Price difference tends to be more about weight, ventilation and style than protection. I think the highest standard of protection is Snell certified but not many helmets are these days.
You'll need gloves in Winter - fingerless gloves (track mitts) are a good idea in Summer if you plan on falling off on tarmac. Mitts might cost a tenner upwards, gloves can cost anything - a tip is that they don't need to be cycling specific and will probably be a lot cheaper if they aren't.
Track pump at home is useful - and a good mini pump like a Lezyne road drive medium to carry with you. You can get by with a single decent frame fit pump for home and bike IF you can fit one to your bike. Track pump might be £20 upwards - good ones probably more like £30. Mini pump £20 up for one that will work - and most don't. Frame fit pump maybe £20 but only get one if you know it fits the bike.
Not sure what clothing you'd want as I'm not a triathlete but you probably want some bib shorts for training and some long legged tights for winter - if these don't have a chamois then you'll need some undershorts or more likely some cheap non-bib shorts to go underneath. Cycling tops for hot and cold weather - you can wear running tops underneath as base layers - if it's very cold you can stick an old woolen jumper underneath.
If you plan to leave it locked up anywhere public then I'd suggest you need a very very good lock - most people would commute on a hack bike if they don't have somewhere very secure to leave it.
There are probably other things.