You've probably answered your own question.
You might like to think about 1 GByte RAM to work with large graphics. You may also find that the Pentium M is slow when rendering large graphics files.
Since weight(and, presumably, battery life) is not a problem, you might like to consider an Athlon-64 processor laptop. Windows XP 64-bit will be out soon and this gives a much better upgrade path.
For £1000 you should be ale to get an Athlon 64 3200 processor, 512 MB or 1 GB RAM, Radeon 9600 or 9700 graphics and 60 - 80 GByte hard disk.