Are you speaking from a position of knowledge or are you just being cynical? GPs can still prescribe many drugs that are available cheaper over the counter. Medicines for children are routinely put on prescription (free) where an adult would be advised it's cheaper to get OTC. I couldn't see sildenafil being any different. Apparently it costs just over £1 for a pack of 4, so it's not as if they're losing money on it.
If a GP Practice, or a Clinical Commissioning Group can cut money from their spend by not prescribing drugs which are available OTC then they will do so. A lot of GPs no longer prescribe things like hay fever tablets, pain killers, skin creams.....
Theres a limited fixed amount of cash available each year for each person on a GP's list, and refusing to prescribe anything the customer can buy saves a lot from that budget. And yes, I do have knowledge. I spend a lot of time in GP surgeries as one of my jobs.
You can expect all those free scripts for OAPs and children to go. For instance, round here there's a total ban on supplying OTC painkillers on script, even to OAPs who need them for arthritis.
The NHS is bust, it really is