The Land Mattress is really bad in it's current state. Both it and the Sexton need buffs so I'd rather not see them anywhere else until they are improved.
The Land Mattress isn't proper shock rocket artillery and has the stats to prove it. When we speak of "rocket artillery" we mean a platform that dumps a large amount of wiping damage in a small-ish area in short order. And their effectiveness is directly related to their ability to do just that without dying. Pwerfer is god if it can get close enough, double Pwerfer barrages are insanely lethal. Calliope is behind them, and is being nerfed SPECIFICALLY to reduce shock potential. SzF is much nastier vs bad players and/or massive team weapon lines who don't react/can't move fast enough. Again, shock = bleed (KILLS)/time/area. The Land Mattress just doesnt do that. Does not meet the criteria of shock artillery. The damage is not bleedy, nor quick, nor focused and is easily counter attacked. What it does is generate cover and deny an area. It takes 12 seconds to fire 30 rockets in a gargantuan area. Thats 2.5/second. The Pwerfer takes 2.625 seconds to dump 10 rockets, 4 per second. Calliope does 18 in 4.5 seconds, also 4/second. The only one slower is the Katy at 1.3 per second, but the first 4 rockets come in a super short period, about 1 second. Notice that magic 4 Rockets per second. Also, the Land Mattress's rockets are all weaker than any other rocket arti with significantly more AoE drop off. It does all of this with the single longest barrage cooldown in the game, 140 seconds. It can be fixed, just needs some tweaks.
Funnily enough, the new B-4 IS shock artillery. Each shell is just.... HUGE with no real warning.
Math on the Sexton shows its... alright. The biggest change it deserves have is what all of UKF needs: lower pop cap vehicles. Its pretty affordable, does reasonable bombardment into FoW. It just has the super small 80 dmg radius of all 25 lber shells, and British explosive projectors. All UKF equipment has slowly been reworked to do less bleed, they just need appropriate balance.
Its also the only arty that can effectively double barrage, which is extremely nasty with 2-3 Sextons. Though I would prefer the Valentine able to dump the call in and run, instead of sitting for 3 seconds.
I would cut the pop cost to 11 or 12. Maybe add a shell to emphasis its smaller, easier to shoot shell.
UKF needs some across the board pop cost cuts to its armor due to the loss in power over time. If the Churchill would be problematic, just cap it to 1.
Suggested Changes:
Land Mattress Near AoE->.35, Land Mattress WP Barrage made INCREDIBLY deadly and reasonably accurate. I know there is properly lethal WP in game, double the tick time. Munis for an actual shock artillery strike. CD for both to 120 or 110 (I suggest 120), in line with other rocket artillery. Scatter of WP -> Angle:10 Max:15 Offset:0.1 Ratio:0.156. I may have copied that directly from the Pwerfer. Also, instead of giving the Calliope a higher min range, just decrease its Mid AoE value to .35 from 5. That means 2 Mid shots are survivable.
1 pop off Cromwell (its between a Sherman and P4), 1-3 pop off Churchill (balance Croc independently), 1 pop off Comet (maybe), 2 off Sexton (it isn't anywhere near a priest or a real Howitzer in barrage value), 1 pop off Firefly. The massive pop reduction to the Bofors was INCREDIBLE for giving UKF a some-what affordable AA option.
The UKF population numbers are from release when all the armor was over-tuned. Now its just mildly suffocating due to all the required specialized units, especially with the need for infantry AND extra engineers for snares. Otherwise, move snares to IS OR make Royal Engineers good enough to replace an IS in build order.
I can't speak for the Centaur, I don't use it enough.