If I had this diagnosis, my thinking would be along the following lines...
The chances of a single met is quite small. Most likely, it's either not a met, or there will be more micro-mets (mets too small to show up on scans). So it's very important to get it investigated first as far as possible. A second opinion on the PET scan might be useful, as might some other form of more detailed imaging of the location (such as CT or MRI).
If it's no longer thought to be a met, then a focal therapy could be considered if the known cancer in the prostate is within the parameters for a focal therapy.
If it is thought to be a met, you are a high risk patient as it's already spread. Micro-mets in and around the prostate are a significant risk (which is where it would most likely spread first). I would be looking to do radiotherapy of the prostate, seminal vesicles, and pelvic lymph nodes, to take out the 'mother ship' of the cancer as you put it, and any micro-mets spread to the most likely locations. This would also come with hormone therapy. That is likely to be available on the NHS, as it's a fairly common treatment for high risk patients. Then I would be looking for SBRT (focused/stereotactic radiotherapy) on the known met. This doesn't have a high cure rate, but even when it doesn't cure, it's beneficial in putting off further treatments. This is unlikely to be available on the NHS - it's only available on the NHS if the met isn't discovered until after the initial radical treatment has been performed. (Yes, I know this restriction doesn't make much sense). However, you could probably pay for this privately, or if you are being treated at a main cancer centre which also does research in these areas, you might be able to persuade them to give this a go on the NHS on the basis that the standard NHS imaging done before this procedure would not have found it.
You really need to talk this all through with a competent oncologist with a wide range of experiences.