Unless you're going to do this yourself (and not comply with Building Regs), the law usually requires all replacement boilers to be condensing under Part L of the Building Regs, so a Gas Safe fitter should refuse to fit a back boiler now.
There is a points based exemption scheme whereby you can fit a non-condensing boiler in limited circumstances.
But the old back boilers are horribly inefficient, you will be paying about 50p/day just to run the pilot light, and you also have the whopping big hole in the wall for the combustion ventilation. Plus the not insifnificant risk of carbon monoxide poisoning.
My Baxi Bermuda went to the great scrappie in the sky a couple of weeks ago, 2 days work to put a new condensing combi in the kitchen, connect up the pipes under the hall, rip out the old tanks, and block up the big holes in the lounge wall.
Routing the condensate drain may require some cunning; I have a plastic pipe running along the window sill inside which makes the kitchen look a bit like a porta-cabin toilet, but one can't make an omelette without setting off a smoke alarm.
