I have used Superdrug and Lloyds Pharmacy - as mentioned before, can be cheaper (and less public) than over the counter
Personally, if I'm putting medicines in my body I'm sticking with what I can get from registered practitioners. I used the on-line doctor from Lloyds Pharmacy to order Sildenafil after completing an on-line check-up and questionnaire. It's up to you if details are passed to your own GP or not. You can have them delivered (in plain packaging) to your home or you can pick them up from any convenient Lloyds Pharmacy store (in standard prescription wrapping).
As to usage, I was 40+ on my first use and was looking to to prime the pump for an overnight punt and I took one 50mg pill. I had a good time but couldn't say whether it made a difference. The next time, I took two 50mg pills a felt a definitely throb that kept the home fires burning. Side effects? I felt a quickening pulse and slight headache.
As others have said, hydration is the key but it's a tricky balance between needing to pee all night and keeping it up all night. I swapped to 100mg doses but never took more than one. I use them very occasionally, just to ensure that what goes up, won't go down on the few punts that funds permit these days.
The last time I took one, it was with a meal and fluids, just under an hour before the fun began. The fun lasted around two and a half hours, not full on porn star hours, but arousal and foreplay and sticking it where I could on and off without flagging. After a toe curling finale, I was exhausted so after hydrating again, I went to sleep.
If this was a review of the O/N, I'd comment on my disappointment that there was no continuation in the morning as my companion was up and out without putting any more notches on the bed post.
But it isn't, so back to the pills and I woke with more than just morning glory and felt the warmth for the rest of the morning off and on. Every time I gave the little fellah some attention, it stood to attention. I must have go the hydration right as I had no headaches but I made sure that I drank plenty of fluids the next day as well.
Hope that helps. EM