I read a forum post asking how long it takes to receive a Giffgaff sim, it's usually up to 7 days (it may be quicker) and sent out by second class post.
If you're impatient like I am you can usually find them in Sainsbury's local stores ( open from 7 am to 11 pm) They are probably on sale at numerous other stores also. They cost 99p each.
I do not work or have any connection with Sainsbury's apart from getting milk and chocolate from them.
Hope this helps.