i find that KScotts Pax Britannia X mod is better than Proffesorspatula's as it makes the game harder, reskins the RB and adds more provinces to Britain. It also puts a fairly large celt army outside Eboracum(York) which you have to get rid of, as well as giving the celts more units, more family members and more money....its a pretty good way to play the Romano British imho, you can find it
at
http://www.twcenter.net/downloads/db/?mod=741

And there is an interview about it here
http://www.twcenter.net/forums/showthread.php?t=37939

After that shamless piece of plugging here is what i did to take Britain and Eire

It does make that stratgey differant:

Start off by recruiting the merc gallowglasses, sarmation horse archers and merc vetenarii then take all the units you can spare from London.

March down to Alpins army and attack it, this is quite hard for your small army with only 3 legionaire units but it is doable.

Sarmation Archers kick ass against Pictish Spearmen when the spearmen form a schiltrom they become more vulnerable to archers, Hounds of Culaan and Gallowglasses are what you really need to worry about.

Once this army is beaten, stay in York and raise a bigger army to go and take Dal Riada, once that is taken (and its not easy :p ) stay in Briatin and clean up the Celtic armies, once they are out of the way head to Ireland and take the two cities their, and that will earn you the land of Britain and Eire :D