The prices are due to change 2nd April and a SASE weighing:
0-20g: £0.88p
21-40g: £1.28
41-60g: £1.88
These are letter rates. Sometimes a board-backed envelope gets counted as a "small parcel" in which case it would cost £3. But you should be fine getting letter rate.
Thanks so far, kimmylee!
I got around 28 IRCs here, but I don't really know how to handle them, how much to include for which size and country... It's soooo confusing...