have you checked your king isn't cut off from the rest of the empire sitting in Britain with no sea connection, or a long way away from the rebellious provinces through land connections? IIRC the distance from kingy to a province in terms of number-of-turns-to get-there has a big impact, so maintain good sea power (after all, you are British) and keep him in a province with a port.

I got a question about rebellions. I know that you can engineer a rebellion but can you also control the size and type of the rebellion? This would be very handy to build up some command stars for my crap heir.
You can a bit. You get more rebellious troops if the garrison they are rebelling against is bigger. Of course the trouble is a bigger garrison also increases happiness, so you will have to set taxes sky high, destroy all happy buildings, give the governorship to the worst candidate you can find, and hope they rebel against your larger than normal garrison. Doing this in a naturally rebellious province (Portugal, Scotland, there are others) would also help. if you were really keen I suppose you could set up an island as a rebellion farm and trigger rebellions by moving ships around to make and break a connection with the king. You'd have to be careful not to get your heir stuck on the island with a destroyed port, your king dies and the heir inherits, and your whole empire go up in rebellion though...(although in a way that would be mission accomplished)

Contriolling the type of rebellion is harder. If you use a recently conquered province you are quite likely to get a loyalist rebellion which will be bigger and have much better troops than usual. I don't know any way to make it more likely that a "vanilla" rebelion will be catholic bandits or what have you (except Rome and the papal states usually seem to go that way. Once I spammed Rome with imams and got a muslim rebellion when the pope eventually respawned which I thought was quite funny, especially as it kicked his bottom, but I was bored at the time...)