You should try to play agressively early to slow them down. If you let brits build their structures on your side on the map, you have done something horribly wrong.
That aside, as you should get mortars of your own - ISGs - and keep moving them after every barrage so the brit mortars can't snipe them back. If they brace, move volks into position and throw a flame nade as the brace wears off.
Also, Fortification is hardly the best doctrine; it's very useful in some situations but it relies too heavily on static structures, which the brits can easily wipe out with their artillery (and God help you if they have Advanced Emplacements; the counter-battery will smash your guns to pieces).
All in all its a similar problem to the one I described in another thread.
Im trying to play aggresively. But if my teammates can not support me and deal with brits too, AND THERE ARE 2 BRITS, not one, I cant do anything. First problem - they have Vickers. Its not my sde of map, Its center somewehre. But in fact - Im loosing center very fast. If it would be soviets or americans - I can counter them with ISGs. But not the Brits. Their Mortars are killing crew of my IGSs very fuc.king fast. So ISGs cant counter their Vickers. And ISGs are not usefull against British mortars - they almost cant even damage them. They are building not 1 mortart but 3-4-5. Really! When there is 2 brits its always like that. With vicekrs there its impossible to come there with infantry. They can kill my squads even without Vickers - Mortars are killing everything. Ok, I managed to come close to one Mortar so I can use flame grenade. BUT another Mortars always are in different place - So they are killing my squads when they are trying to destroy that one mortar replacement. And after... After Brits already control more than half of the map. So they have advance. When I finally will have 1 Panther - it will not help so much... Its just too late. They will have Cromwells, AT guns, probably more infantry etc. Even comets