Products online are just as safe as from the vet, but as you've discovered, half the price!
I've used all the above, plus others to look at (I've used) www.wormers.co.uk , www.petprescription.co.uk and www.vet-medic.com
It's worth shopping around as some online companies don't charge postage for orders that include wormers, and some are free if over a minimum purchase value. I expect to pay around £13-£14 for a 100ml bottle.