Ok...with a little more info, the idea of the British I6 with EFI and an OD trans makes sense.
The V8 LPG option might also work, but I don't know enought about it to have an opinion.
20 mpg in a 49 with a V8 is possible, but it wouldn't be easy. I still think that 15 mpg is more realistic with a carbureted V8 unless most of the miles are highway with an OD trans.
Most aftermarket cams are made with performance in mind, rather than mileage. A stock cam will probably be the best bet for a mileage setup. Remember: most performance parts are designed to improve the flow of air and fuel thru the motor which generally means using more fuel. While the motor might run more efficiently from an airflow standoint, it won't necessarily decrease the amount that's used.
Another "outside the box" option might be a Vortec TPI V6 from a 96 or so Blazer along with the transmission and rear end. You'd have to do the computer thing, but it's been done before. Get the whole package from a wrecked one (They might not be all that common in the UK, tho) and use the whole thing. My wife has had 2 Blazers like this (she currently has a 2004 and loves it) and consistently gets around 20 mpg, although she doesn't keep track of her exact mileage. Most of her driving is in town.
Just some more thoughts.
