If you have a warranty, why are you talking about doing it yourself? If it's under warranty, they will do it for you. If it's not under warranty, there is no warranty to be voided.
I'm assuming that you have a Titanium PowerBook G4. Without any more information, I can't give you much in the way of details.
Forgive me for my lack of certainty here, but it's been a while since I've been inside a PB G4.
If I remember correctly, most TiBooks have the DC-in jack on the motherboard. Some, however, do not. If the jack is not on the motherboard, you don't have to replace the motherboard to replace the jack. That's it.
I don't know about places in the UK that you could order from, but I know that Preowned Electronics is great here in the US. I don't know if they ship to the UK.
http://www.preowned.com