Nothing you do requires a fast processor or lots of memory.
Storing music/photos uses no processing power, word/excel uses only average processing power/memory, web and emails take little processing power/memory, and downloading music depends more on your broadband speed not your laptop
Your broadband speed will probably be your biggest inhibitor, not your computer speed.
As a general rule
- avoid Intel Celeron or AMD Sempron CPUs (they are the slowest)
- go for 3Gb of memory or more if you can (you probably dont need it but most laptops come with it nowadays anyway - 2Gb will probably be enough though)
- Minimum 250Gb hard drive, bigger if you can (but get an external hard drive of 500Gb or 1Tb to give yourself loads of storage space)
The most basic laptops cost around £300 nowadsys (give or take). These tend to be Celeron/Sempron laptops with 15" screen from supermarkets or discount web sites.
For £350 or so you get a bit more, maybe an Intel Pentium Dual Core 4300 or 4400 cpu with 3Gb memory and 250Gb hard drive.
Then we get the £400 laptops that are just a bit better all round, or maybe a better make, or with HDMI out that sort of thing.
If you dont lug it around a lot consider a 17" screen (good for looking at photos / movies, but they are heavier if you lug it around a lot).
For what you want to do I would say a £350 to £400 laptop would be fine, maybe a bit more for a 17".
Here is a starting point to see what you can get for the money, a Dell Inspiron 1545 at £379 (not saying this is the best, but it is a starting so you can compare with other laptop models.)