Since relic refuses to give them a mobile mortar your basically shoe horned into mobile defense, even then the land mattress is very mediocre at best.
Maybe the sexton is going to be better now that its getting buffed (If i remember correctly).
If they are insistent on not adding the mobile mortar then a pack up for resources back is almost necessary for the faction.
I don't see a problem in implementing Planet Smasher's Artillery Pit, at least just for testing purposes. Their mortar pit model won't go to waste and won't be put in a doctrine or whatever and the Brits will have a more balanced and practical mortar which is proven to work.
Apart from that tho the Land Mattress costs fuel and is slow, and is pretty easy to take out as I've experienced.
While the Sexton is in a basically team game commander that no one in hell is going to choose or at least call in a Sexton in a 1v1 or 2v2 since they're small maps and you're already spread thin and low on manpower as a Brit.
The other options for a doctrinal mortar were to add it to Arty Regiment to make the commander useful in small team games or give it a mortar HT, the mortar which was added to the air resupply operation for Tac Support Regiment and the new commander coming out around on Christmas having a mortar team as well.
Apart from that I don't see which other doctrine could be given a mortar besides maybe Special Weapons (to replace the Tank Hunters?) but it's sad that we're trying to think of ways to fill commander slots with something that can be done much more easily and with less headbutting, commander slots which could be used for something else entirely.