Agree that the extra £300 is very steep!
The main repercussions of jailbreak and unlock are this:
1. Apple will not give you warranty service if you take them a jailbroken phone with a problem, they do not like jailbreaking. However, the good news is that you can easily unjailbreak and put the phone back to "out-of-the-box" condition via iTunes. And if the phone should totally die, ie it won't turn on, then Apple won't know it's jailbroken anyway. Just remember to take your unofficial SIM out before taking it in
2. Apple are continuously trying to close jailbreak exploits, so we often find that we can't apply the latest firmware update immediately - but eventually someone finds a new exploit. This forum is the place to come to keep abreast of updates.
3. You can have a lot more fun with your phone once it's jailbroken - there are lots of ways to theme the phone, to give it new sounds, to do things that Apple don't want you to do (eg jailbroken phones could use Skype over 3G when standard ones couldn't)
4. Jailbreaking and unlocking is very easy, and there are detailed tutorials on how to do everything at
http://www.iclarified.com/tutorials/iphone/index.php
One final thing to say - the very latest 3GS's (ie the last few weeks' manufacture) have new boot-up software which means that the jailbreak is "tethered". The implication is that if your phone is powered off totally after jailbreaking it, you will have to reconnect to your computer to run Blackra1n again when you power it back on. That's the current state of play anyway, though that will probably get sorted out before too long.