Bone scan?

I wouldn't be concerned. I don't really know the criteria they use, and perhaps more importantly for the NHS the cost of different types of scan.

It is always possible cancer has spread but less likely in the early stages, so your onco probably decided it was not likely enough to justify a scan.

An advantage to the NHS of doing a scan early is that, if it has spread they can rule out a lot of expensive curative treatments (it's also an advantage to the patient, because he won't suffer pointless interventions, though sadly he is incurable).

Assuming you have a low stable PSA there is no point in looking for problems. If you have recurrence manage it when it shows up in real life, don't go looking for trouble.