(You don't need a board-room, by the way, the kitchen table is fine for the moment.) There should be at least one director and a company secretary. The company secretary (not related to a secretary you might find in an office), has certain legal obligations and duties. These are not particularly arduous, but you might like to pass the whole thing on to the company who's advertisement you originally replied to. For an annual fee (about £35 at the time of writing), they will take the whole irritating business off your hands, and act as company secretary on your behalf.
For a similar piffling fee, they will also offer you the use of their address as your company's registered office. Every company must have a registered office in order that the long arm of the law can track you down if you're naughty. If your address happens to be "Flat 6, 14b, Cherry Tree Close", then you might find this address a bit embarrassing on your note-paper - not quite the right image somehow! However, a smart, central-London address as the registered office looks pretty good. You can't use the address as an accommodation address though, so you will have to put your actual address on the note-paper as well, or you could employ the services of an accommodation agency if your own address is really terrible.
An accommodation agency is simply an office with a smart address. For an annual fee, they will let you use their office as your own address, and will forward all mail to your real address. It has to be said that accommodation addresses are usually used by people with something to hide.