If you can deal with the looks then the 640i GC is a relative bargain.
I presume you mean the 640i Gran Turismo, the 640i Gran Coupe is a great looking car
It is, however, a very good suggestion if you can deal with the looks. It doesn't exactly look THAT bad and if it looked any better it wouldn't be quite the bargain it often is.
The problem with the 5 Series in the UK market is many of them are really quite poor specification as people prioritise things like 20 inch wheels over more useful specification like memory seats and the upgraded headlights. So most 540i you find will be missing one or more of the nicer luxury features you'd expect on a car of this type, especially given the car market at the moment you can expect to be paying over £30k for really quite an old car.
But with the 6 Series, the upgraded headlights, memory seats, panoramic sunroof etc are all standard fit across the range - they all have these items. So its much easier to find a really nice specification car. It is also absolutely huge - it has greater luggage volume in the boot than the 5 Series touring.
So really, the only reason not to buy one over a 5 Series Touring is if you really, really can't stand the looks.