A couple of thoughts,
Name a few examples of tones that you would want to get from the amp.
Firstly you will not get a lot of clean headroom, so to get modern tones (at any sort of volume) you may have to be be using a pedal on an already dirty amp. Some amp / pedal combinations sound good like this some don't (and prefer a clean amp)
As previously said most of the small amps go for vintage tones some of them are :
Matamp Minimat (7 watt I think)
Epi Valve Jr (5 Watt)
Fender Champ 600 (5 watt) Supposed to be about 120 pounds on release
Z-Vex Nano Head (0.7 watt)
Ampmaker SE-5A (5 watt)
Orange Tiny Terror (14 & 7 watt as I remember)
Even the Nanohead @ 0.7 watts is a bit too lout to run at full volume for bedroom practice in my circumstances (though that was through a marshall 4x12 heheh)
If I was looking for a 5 watter I would personally look at the Minimat (it has an attenuator built in)
You could also go for a 15 watter and fit an L-Pad attenuator from ampmaker
http://www.ampmaker.com/index.asp (once he gets his house move over with) which costs about 7 pounds.
Rob...