Do I need to pay capital gains tax? What are my options?

TM1976

Depends whether it's a capital gain or income from trading.

Best to get advice from an accountant on this to get the most tax efficient transaction and to work out whether you are trading or not. An intitial consultation should be free.

Cook_County

You also need accounting advice on the optimum accounting period, the Annual Investment Allowance for capital assets (eg computers).

Separately you need legal advice to check if your employment contract permits having another job.

TM1976

Jimmo gives some good advice here, it's always useful to understand what the taxman would do in this situation.

I also agree with his judgement that you are trading but as you say yourself it's difficult to say without knowing the detail. What's also true is that your argument that it's a hobby wouldn't hold weight with the taxman. In fact there are quite a few businesses that work on the model of investing a lot over a long period of time and getting a single sales (eg pharmaceutical development cos work like this).

What's at stake here is a few thousand £s so it's worth taking professsional advice.