I hope you are'nt doing your own wiring now that Part 'P' has come into operation??!!
Yeah you can get dimmers that can handle 500W.As suggested before,try at an Electrical Wholesalers,(plenty in the Bedford area) rather than B&Q,Wickes etc.
If you need a two gang switch for the hall/landing,you may have to buy a "grid" switch.(i.e one ordinary 1 way and one dimmer)These are switches that you can make up to your own needs.You buy the front plate and yolk or grid,then fit in whatever switches you require.Crabtree or MK make these.