Well...1/2 mil UK CAA VFR charts usually retail for around £13-13.50. I guess Transair's price derives from the extra % they put on anything they sell and then the cost of p&p.
Options?
Try a flying school/airfield shop near you. You should get them for retail price.
Phone Norman up at
www.pilotwarehouse.co.uk. He's a very, very nice man. The website price is a bit steep but try a haggle.