Ok but you can only pay in £5k say (dependent on age in a cash ISA) in per year
Matched betting isn't a lot of work tbh, I spent maybe an hour a weekend and sometimes a bit during the week doing it, started off with £200 and my float average for betfair was around £500 so throughout a year. With free offers alone (not used returning offers until last week with a £20 no lose bet with betfair which I made £15 off) I made all the profit of ~£600 (not including offers like Stan James casino offers which were also returning a healthy profit of £30+ monthly for a 50p loss on a qualifying bet), there wasn't a huge amount to do time wise, a lot of tools/comparison services are available (one of the people I work closely with at work is friends with the creator of bet72 comparison service) and the community in general on places like MSE is very knowledgeable and helpful
say I invested £1000 (it would have been around that in total with withdrawls/deposits across places) I made £500 or so (still got the spreadsheet for it with all the transactions). 50% interest