I know quite a few people import high strength supplements such as vitamin D from the U.S.
Most goods to the value of £18 or under can currently be imported duty free.
BUT, I just found out in the context of something else entirely (I was buying some jigsaw puzzles from Japan) that this threshold is to be reduced to £15 as of 1st November.
So, a sneaky little tax rise for you, that I haven't seen get any publicity, and I'm sure will catch a lot of people out. If you used to get as close as you could to £18, without going over, you'll have to remember it's now £15. This will mean some things you could formerly buy without duty are now off-limits.
It's not just the duty itself, but IF your parcel triggers it, Royal Mail will slap an extra £8 handling fee on top, which is the real killer. So you'll have to pay duty AND the £8. Makes it not worth it, for most small purchases.