Costs are what you see - rent / rates / maintenance of office space, fixtures and fittings, IT (plus support), connectivity, office supplies, tea, coffee, admin staff, etc. In a bit over three thousand sq ft you get a very nice office, good trading atmosphere, and about 40 x 1,600mm trading desks to split the cost over (while getting any scale benefits possible, though custom nature of outfit will mean these don't apply for everyone).
In terms of how that is covered, the simplest way is to pay a basic monthly rent for the use of a desk, chair, PC and phone and consumables. Of course, we can put together a complete market data, trading, brokerage and custody package if required (we can even set up an entire hedge fund structure if you can raise sufficient assets).
But you can bring your own infrastructure and just pay for the desk. You can even bring your own IT, including servers, etc, and maintain them yourselves (if you like) - the comms room is huge.
Its independent of any supplier and you can shop around for anything you want to ensure you don't feel you are being ripped off on any part of the deal.
Its a trading room, bit like a serviced office, only customised and supported like you would find in a bank or hedge fund (within reason - its not Morgan Stanley). As its in Harpenden, if you commute into London regularly what you save on the season ticket will probably cover most of the office cost...