There is Needlecraft in Hemel Hempstead, that has in the past had a lovely selection of dress fabrics and they also do some P&Q fabrics, knitting yarns and patterns, and an excellent array of habby.
Next there is or was a place in Dunstable called My Sewing Basket, whether they are still there I do not know.
Even further away in Princes Risborough in Buckinghamshire there is Sally's mainly a P&Q shop that stocks of lot of Liberty fabrics for P&Q, but some of course can be used for dressmaking. She also has a reasonable amount of habby and sells some knitting yarns.
For a bigger place there is the Fabric Warehouse in Packet Boat Lane which is between Uxbridge and Yiewsley but I have never been there.
And finally I believe there is a place called Fabricland somewhere around the Greenford area - again I have never been that way.
Whenever I go to somewhere different I put into a Google dressmaking fabrics in such and such an area and then see what comes up, but as you know so many places have now gone it is so difficult to find anything you like unless as BrendaP says go to Goldhawk Road, or possible take a trip up to Birmingham where they have several fabric shops close to a market where there are more fabric stalls.