Hey Sophie :)
I have found my favourite in GOSH X-ceptional wear foundation (only available at superdrug) but I think it may be too heavy if your just looking for a light one. Maybelline are really good for foundation and I would say that the Dream Satin liquid one is the best from them. I also know that there's a new Max Factor one out.. Weightless something or other. I've not tried it but it does sound really good. Personally, i'm not too keen on Rimmel foundations.. I have however used their translucent powder for years!
As for applying the foundation, I usually use my fingers cos I think it easier to blend as you've got the heat from your hands. If i'm going out though I would use a foundation brush.. I wouldn't like the hassle of cleaning it every few days lol.
Also I think if you're just looking for a light foundation you should steer clear of primer. It is mainly for smoothing out fine lines, to even out skin tone etc. I have only ever found one primer that actually does these things but it does make your face feel quite caked.
Hope that helped you :) xx