depends on the tyre mate, i just weighed one for you with a michelin pliot tyre and it was 20kg on the button, if it's bridgestone with the heavier sidewall that may take it up to 22kg but should be no more than that.
personally i've been lucky with royal snail - if you can sneak the parcel under the 20kg then it can go their standard parcel rate which is only£11 for 20kg. I'm guessing these will probably be more so you could be looking at around £50-60 for their parcelforce 48 hour job.
If you can put it thro a business then TNT are quite cheap - got a friend with a business?
i hope that helps
Martin