I recommended 28T in the previous post since this gives you the option of swapping one pulley for a smaller one to change the ratio. It's best not to go below 15T as bending the belt round such a small radius stresses it needlessly.

I've used Bearing Station and Belting Online recently. Mixed feelings for both of them - they're both exceptionally slow to despatch and with bearing station even when I paid more for fast delivery they were still slow. Not tried faster delivery with Belting Online. In the last order, which was about £100 worth they initially sent one incorrect pulley and belt, but they did replace the belt once I sent the offending belt back. I didn't bother changing the pulley as they only sent the wrong width pulley, which was still suitable.

The pulleys from bearingstation come just centre drilled so you can drill/ream them to size. Most belting Online pulleys seem to be 6mm bored, so you would need to change them anyway although they do offer a service to bore them and add grubscrews for you. Since you've got a lathe there's probably not much point, but you would need a reamer or exceptionally small boring bar to get an accurate 8mm.

Anyway, I'd go with Bearing Station as once VAT is added they are cheaper than Belting Online.