Larger wheels and tyre will affect the overall gearing and thus your speedo and odo display.

Go to

http://www.alloywheels.com/TyreCalc.asp

and work out what is feasible.

With a larger rolling radius your engine may not be able to "pull" your vehicle uphill or underload unless you are prepared to do a LOT more gear shifting to keep the engine "on cam."

If you go seriously large without changing the diff ratio you may never get out of third gear...however if you live on the Fens then this may not be an issue and your fuel consuption should improve once you get a cruising speed that the engine can maintain with the original gearing and new tyre/wheel combination.

Also be aware that larger wheels and tyres will alter the handling/grip as well as the ride partly due to the increased air cushion you will be riding on, and partly due to the increase in unsprung weight.

You may also require a change of dampers a longer stroke and softer setting, adjustables would be a good move then you can tune things to your preference to get back a comfortable ride.

Cooper discoverer STT are a good tyre at a reasonable price, they last well on road with mine lasting over 60,000 miles and still in good condition.
These are good in mud, water, and over rocky terrain.
Been down the road of remoulds, it works out cheaper in the long run to buy a good tyre to begin with, these are a reinforced tyre and to date i have had no punctures unlike the remoulds.