Thanks chaps for the responses.
I may explore some of the cheaper options. I guess Superdrug has suited me as I can pick up in person from the local pharmacy counter, and this is very reliable and safe.
I know other providers have options to deliver to e.g. local post offices but they presumably use 3rd party delivery services, which I'm less inclined to trust. I once ordered from Lloyds online and, instead of delivering to my specified post office, Evri sent it to my home address. The packaging wasn't totally discreet either as there was a little Lloyds sticker on the outside.
Plus any saving in cost per tablet is much less than the punt I would use it for.
But it's interesting that there are two versions of Cialis, identical in all respects, but one costing double the other. I wonder whether the fact that the dearer one needs a prescription is the reason. In this case at least some of the cost is charged to the NHS, so the pharma company can get away with fixing a higher price.