For that size and weight Royal Mail recorded would probably be the cheapest
I think the max may be 10KG though, which I think your just slightly over
They look very good - would cost £9.99 fully insured.
Taking it to a shop is attractive because P2G insist on it being collected. They say that is a better service and it might be for some but it means that I would have to take a day off work and sit in all day (my employer doesn't allow collections from work).
But they only allow upto 10kg so they probably wouldn't take mine.
Cheers,
Nigel