Mob-Dev are based round the corner from me.
They're reasonably priced, but you can buy broadly the same things from ebay for ~60% of the cost. Cheapest route to get one worked out at about £125 for antenna, cable run & internal repeater unit.
You need to decide if you want one that does every network, or just one of the two main GSM frequencies. One that sorts out the 3G signal too is more money as well.
As said above, they're illegal to operate in the UK without a license as they are repeaters. I very very much doubt that you'd get caught out though.
I didn't bother in the end, and instead changed to a network that did give reception where I needed it – are you sure none of the networks give reception in your office?
Unless your phone has an external antenna socket, there aren't any other options apart from the SureSignal boxes.
Drop me an email if you want any info.