Just done an interesting exercise - compared the price for the last (new) order I made, from Britannia.
Total order: £239.91 (including 2.00 postage), of which "Classical": £182.96
I tried getting comparison prices from Amazon.co.uk, .com, .de and mdt. MDT had all 24 discs/sets (included 1 9disc set of Liszt piano and 1 5disc set of Beethoven symphs). Amazon.co.uk and amazon.de had only 18 of these and amazon.com had only 17 (a slightly different set). Of the 13 titles available from all sites the prices were:
Britannia: £111.98 -> £113.98 if I include all the 2.00 postage
MDT: £186.50 (post free)
Amazon.de: Eur 280.42 -> £191.28 (unsure about postage or possible VAT adjustments)
Amazon.co.uk: £204.87 (post free)
Amazon.com: $276.78 + $36.36 postage. AFAIK this would also incur about 22% duty + VAT. At £1 = $1.78 -> £214.62!
This order was of course from titles available, and at a good price, at Britannia, so it does rather self-select, but was still instructive. If I'd taken an MDT list, Britannia would probably show up quite a few missing titles, and timing it when sales were on would show them much closer. And MDT still beats all the amazon sites - even with none of the discs currently at sale prices!