A slavebox is a UK term for a new build flat/apartment, sold as luxury but short on space, sold for a huge amount of money due to unchecked property inflation. Build quality is usually poor, with thin stud walls allowing you to hear neighbours fart. Many were bought by amateur property speculators off plan, hoping for instant profit.
A slavebox is so called because after paying the mortgage there the buyer has little disposable income left, thus becoming a slave to the bankers.
Man1: John can't come out tonight as he's just bought one of those new apartments - toilet's in the kitchen and he can barely fit a double bed in the bedroom. Cost him £250k!
Man2: Yeah, they're complete slaveboxes.