A over-all good strategy but you could do it a bit faster, atleast I did...
1) You strip all not frontline settlements of all but 1-2 units and send them to welsh border, this includes stripping places on border w/ ireland because 1, there worthless, and 2, they will more than likely attack norway, but do offer peace and trade anyway.
2) you Zerg ireland with the troops stationed there. Now this will require some superb planning, strip all places expect to 2-4 units, strip ALL units from the forts, and join them up and strike north and take the Irish settlement there, with some good planning this is doable, altho it will not be without causlties.
Next after you have all settlements North of you, Reinforce your settlements and send any surplus troops to caevroon, (bad spelling,sorry) I believe its the welsh capital, more often than not it only have 2-3 if not 1 units in it, build all you can in 1 turn and attack, unless u are a dimwit or attacked when it is well armed then it should be an easy victory, assuming you'r not attacking with 2 units, if that is all you have reinforce a current army.
3) at this point you should have delt with the Welsh, you don't need to worry about there armies in the field unless there near a settlement, just take all they have and they disappear or become small rebels.
4) it should be turn 10(if you are as good as i an >.o) to turn 20 if you like to take your time. Its time to try to make peace with ireland, and get trade, if not then go all out on them. Deal with there castles first to stop them from getting good units. If you can get trade, then you turn north, you want to, of course, get peace and trade with norway, once you do take all the poorly guarded citys and you should soon draw the attention of there big armys that by now should be on the islands, after taking two or three of there petty settlements big armys should start heading your way, find a fort, sit in it, and send a spy around to find near by armys that are one turn away, you dont want to get stuck in the open where it becomes a 880 Vs. 3259 -.-(me vs them) anyway. you should be pretty bankrupt right about now.. so its time to attack and sack all citys you can find. leave the castles to norway, who should have some breathing room now, get peace with scotland and sit tight for a while, dispand exess troops, and let your economy grow. about the time edwards gets back from his crusade, (if you let him go, if not then when wallace arrives) you should have a stable economy, wallace has a worthless but trained army, try to ambush him, if not then just kill him, its easy, after that, leave scotland some places so it can continue battling norway, that way they keep eachother weak. Now, buy all the troops you can and send them to ireland, this is a 3-5 turn campaign. ireland has 3 settlements. armys of 10 should suffice to take each of them, and take them all in the same turn, so you have no possiblity of counter. After elminating Ireland all thats left is to deciede if you want to take norway or scotland first
Sorry for the wall of text, i kind of got into this a little to much lol