you get what you pay for, if you specify you want the pipes chased into the wall then be prepared to pay a fortune. also, if its chased into a wall or hidden behind skirting they will probably use small bore pipe i.e. 8 or 10mm rather than the usual 15mm, this is fine while the system is new but over time it can silt up and the system will not heat up as well.
its worth noting that a professional heating installer will prefer to site the radiators under a window if possible, this is because it is the coldest part of the room and the draft from the window will then blow the heat over the rest of the room, so while installing the radiators back to back on an internal wall saves on exposed pipework it is not the BEST way of doing it. a lot of heating installers do it this way as its quick and they make more profit then.
some people are quite happy to have the pipes exposed but tucked into the corners or tight to the skirting board as possible and then painting them the same colour as the walls, after a while you don't notice them. or my father in law as put his in white plastic cable trunking which blends in with the skirting board and door surrounds very nicely.
get 3 firms in to give You a quote, ask them their opinions, discuss with them exactly what You want.
beware of british gas though, they do a first class job but they charge a fortune!