Lloyds online pharmacy - https://onlinedoctor.lloydspharmacy.com/
Which is fine if you are happy enough paying a tenner per low-strength tablet
(they advertise 50mg Viagra and 10mg Cialis with usual massive overpricing).
Admittedly, it is more expensive to buy original-brand, low-dosage, as mostly sold commercially in the UK. But even if you buy originals from a reliable pharmacy abroad, they don't come near that.
As an example of what I think it is sensible
to pay, here is what you can expect to find reliable generics from a reliable pharmacy for:
Cialis 20mg (normal strength) 65p a tablet
Viagra 100mg (normal strength) 63p a tablet
(you can always break these in half).
Different strengths are slightly more
10mg generic Cialis 76p
40mg generic Cialis £1.10
Another advantage of generics is that they are available as chewable mint tablets (works faster, leaves nice taste, fresh breath).
But if you don't know where to buy (many online stores are also either fakes or sell poor quality stuff through middle-men) and only punt once in a blue moon (ahhhhh...) a tenner, I suppose, is nothing.