Any man that has ED as a result of cancer treatment is entitled to a proper vacuum pump on the NHS - some CCGs try to get out of it but it is written in the NICE guidelines.
Treatment for ED isn't really the job of your surgeon etc - the responsibility for prescribing (and picking up the cost of such) falls to your GP; although some surgeons / oncos will write to the GP recommending a certain approach the GP doesn't have to comply. Best thing to do is find out whether there is an ED nurse or andrology clinic in your area (some are based at the hospitals in the urology department but others are out in the community) and ask your surgeon to refer you there. If an ED nurse tells a GP to prescribe, it is harder for the GP to refuse.
The most commonly prescribed pumps on NHS are the Osbon Erecaid Esteem, the Farnhurst and the SomaErect.
Note that there is no research to support the idea that prophylactic Viagra helps with nerve repair and recovery of erectile function. The research was about daily dose Cialis but some CCGs / doctors will try to get away with Viagra because it is cheaper.