Retainers certainly aren't the norm these days. With the slightly higher hourly rate, you'll certainly enjoy the very busy months when you'll be earning close to £2k, it can make the winter months lean, and doesn't help if there's an unforeseen problem like a long term tech aircraft or airfield closure, but ultimately the schools want the flexibility of no retainers, and they have the power at the moment.
Perhaps it will go full circle, with a potential shortage of decent experienced instructors, unrestricted and able to tech night and IMC, Ts & Cs may one day go back towards retained positions, but schools will always pay the lowest they can as their margins are already tight.
£20 an hour is pretty much the going rate, ask if it will improve once you are unrestricted, and if they pay you when you send a student solo, as these can add up to quite a bit extra.