Likely it'll be a bit too acidic for lavender, they like a ph of roughly 6, or around there, and the compost you've got's probably got a ph of 3-4. But if you're planting it out, that sounds like you're putting it in the ground, in which case, you don't really need the potting compost at all....

Lavender can grow in quite poor soil, so no compost needed. Make sure you cut it back after flowering or else it will get leggy and brown off.

Thanks for those comments folks, I should have told you the soil I have is very much heavy clay.

Best off digging in a bit of grit before planting lavender then, to improve drainage.

Thanks a mill folks.