@hornetstinger means well but he is rather out of touch with reality.
Back in the world of mortgages and other bills, the last couple of Monoliths went for under £300. You might end up with a less popular colour or something cosmetically challenged, but if Ebay completed sales are any guide, then it is something achievable. One concession you'll have to accept though is "buyer collects". These things weigh a tonne, and few owners keep the box, so the logistics of shipping are a bit of a hassle. Then there's the cost. BK have a deal with their courier firm, but buying shipping as a private seller is a far more expensive proposition. Be prepared to drive.
Given the extra cabinet size and the bass port, the Monolith will go deeper and louder than the XXLS400, but you will have to be a bit more careful about where you put it. However, if you've got the space then it's doubtful you'll need anything bigger or louder in the average British living room.