Being a small tank you naturally think you can get away with all the smaller diy co2 systems, be they yeast based or little cans of co2 via a simple dome diffuser, hower they will still eat well into your £100 budget and from experience not that effective or easy to control /operate.
The forums general advice is to always by a dual stage regulator, with good reason and to use one with a co2 fire extingushers ( both detailed in this section if you look them up.)
However they will come in more towards the £250 -300 bracket, but you can buy a much cheaper single stage regulator and use that with the above metioned 2kg co2 FE.
The FE cost about £25 and a single stage regualtor costs around £50, local source, though cheaper from China, plus some Co2 tubing and a diffuser, well under £100 and nothing more to buy for a year.
To avoid the problem with the single stage ones all you have to do is keep weighing the FE occassionally to ensure you replace it before it gets anywhere near empty, our 50ltr tank will run for over a year on one 2kg so should be fine for your 30l tank. Just make sure you have somewhere big enough for the FE and regulator.
https://www.ebay.co.uk/itm/The-barn...025753?hash=item5484e7a259:g:lQUAAOSwja5eyfUG
https://www.ebay.co.uk/itm/2KG-CO2-...537924?hash=item3fdef50004:g:WbgAAOSw-olaAroz