A lot to look at here. Firstly there is no "Licence" - you have simply completed training. There is no requirement to do so and none required to solo dive. Anyone that wants can buy anything they like and do any dive they like and if you dive alone there is also no one to stop you!
So my first recommendation is to echo
@diversteve and suggest a solo or self reliant training course. Without that you are exceeding your training so really you might as well not have bothered with that in the first place and certainly outside of what you describe as a "licence".
For shallow and close to shore standard kit is probably sufficient - provided you stay there. What would worry me is that with scuba (as opposed to snorkel) that you get engrossed in what you are doing and get too far from the shore or too deep. As a snorkeller you are always spending time topside to breathe and so always aware of where you are. Once submerged it's so easy to get carried along on the current and to forget - particularly if you find something of interest.
I also agree that watercraft are a major issue - there are big problems round Bournemouth with jet ski users.
The PADI Self Reliant or SDI Solo Diver courses will give you much more information and help you make much better decisions than asking a bunch of people on the Internet, many of whom don't know anything about the UK south coast.
Also you talk about BSAC Open Water - there is no such course. If you are in a BSAC club you will be aware BSAC does not allow solo diving. If you have the PADI Open Water the follow on BSAC course is the Sport Diver.
I am certainly not saying you should not pursue your interest - I hope you will do so. I just think the next stage for you is to get the appropriate training and to discuss your particular requirements with an Instructor face to face. That is the whole point of such courses. It will cover equipment properly and give you the knowledge to choose the appropriate kit for the dive. Here on the Internet we like to argue about pony (bailout) tanks, but one might benefit you or it might not, depending on the circumstances. The course will cover that and show you how to use one properly and rig it correctly.
Sounds like a great way to spend a few hours - I hope it works out OK for you.
