Well, first, I'm not sure whether this belongs in the "buy/sell Auction and reseller forum or not, but if it moves there soon, don't be too surprised
In England, th Marshall guy would have to expect for fork over quite a bit of cash, probably several hundred quid for that sort of trade, simply because of the RRP on the products. The Boogie, is twice the price of the Marshall head over here.
I'd reckon have look on ebay and see what both amps are going for used, then use the difference as a starting point for any financial arangements that'd be needed to make with the swap.