OK here goes, ages since I did a kit though. Good for you making headway into a great hobby and regarding temperatures believe it or not 21c is on the 'high' end of the temperatures you want to aim for for fermentation. Once the Barrel has had some time at 21c to condition I would recommend (not alway easy) serving it from a cooler room at say 16-18c but when you move it , it will require a few days to settle again unless you are very smooth and dotn agitate it at all.
When I started I was under the impression that you needed airing cupboard temerature to brew beer this is simply not so. I used my heater twice and that was it - the room that I keep my beer in stays in the 16c - 20c range and I ferment and serve at that temperature everything but lager. Lager required a lower temperature if it's a proper lager yeast something that may need investigation - the lager kits however may well come with a yeast suited to 18c and bitters rather than 12c and lagers i'm not sure so view their instructions for fermentation temperatures.
IIRC Coopers were the kits that i did for lager when I first started and they did an Australian Lager but to be honest the quality improvements from a 3Kg kit will make for a better drink than one bulked up with sugar. Also regarding lager those placcy barrel;s don't really gas well so if you like your lager fizzy you'd probably be better with bottles.
Welcome to Jims and a great hobby and read here for more kit advice.
http://www.jimsbeerkit.co.uk/kits.htm