Last year I got my WATOK licence and am now licenced for both electrical stunning and mechanical stunning and slaughter using a Cash Tool. For an electrical stunner or a Cash tool you're going to have to invest £900 - £1000 which is a huge investment for such a small number of birds. They also need regular maintenance. I have also found that Cash Tools are very ineffective on Cockerels because you can't position it in the right place due to their very large combs, so worth considering if it's cockerels you'd be dispatching. They are both very powerful and potentially very dangerous bits of kit... I would really not recommend using either without proper training. Without proper training you're also at risk of not stunning the bird effectively. If you are still interested in either of these methods then I'd highly recommend getting in touch with HSA and booking onto a training course with them which is what I did. They run day courses, there is a theory and practical session where you are taught different methods and how to make sure you stun effectively. If you pass you receive a WATOK licence. But this will also cost you another couple of hundred.
For your set up unless you really want to expand I would recommend just using manual neck dislocation. It's legal and if done correctly a 'humane' method of slaughter. If it's something you've not done before don't practice on a live bird because it's not as easy as it looks, there is a bit of a knack to it. I personally get on best with the 'heavy stick method'. Be careful not to put an pressure on the birds neck while getting into position by standing on your heels then in one fast motion push your feet down and pull the birds feet up towards you. There lots of examples online. You can do large birds this way and don't need to be strong and it's not as technical as the hand held way.
The HSA website has lots of useful information on different methods and also signs of effective stunning which is important whichever method you choose to go for.