The fuse box (not the make fuse box but real fuse box) was used until late 90's when the name changed as we started to use type tested equipment, and it changed name to consumer unit, which could and still can have fuses in it, and the old Wylex fuse box can use re-wired or cartridge fuse or a MCB, and very easy to swap from re-wired fuses to MCB. However the main reason to swap a fuse box to a consumer unit is the latter allows use of RCBO's, RCD's and SPU. Although in theory you can all RCD protection
as seen here above the old Wylex distribution units, it is not cost effective, that was done before the modern consumer unit was produced around 1992.
So the consumer unit width is controlled by how many circuits and what is fitted the MCB or RCBO are a standard width around 17.6 mm, so total width is controlled by number of devices, a RCD takes up two slots, so an all RCBO consumer unit is around 70 mm shorter, some SPU are double width and some are single so by using a single width one you can reduce overall length.
However the hight of a RCBO may be only 81 mm, but most are taller, and I don't know there is any consumer unit made today which will not take the longer version, so 260 mm high is around a standard height, there may be the odd one which is not as high, but unlikely anything today will fit in the space taken by the old fuse box. The box has these
in it all around same hight but is designed to take the taller version
so very little difference between makes and models in height. And to be frank Fuses, MCB's and RCBO's produce heat, and the consumer unit needs to get rid of that heat, so there is a limit as to how small it can be made.
The cost of a change is labour plus materials, it is easy enough to price materials, but the labour is not so easy, I would say even with the smallest easiest CU change it could not be done in less than 2 hours, but the problem is once started it must be finished, and I have seen an electrician still on site after 8 hours and doing some temporary fix and promising to return next day, actually never seen again, but my point is most electricians will allow a whole day to change a consumer unit as until started often unaware of all the problems. So in the main looking at a days labour charge.
Back in 1980's my labour was charged out at £25 an hour, I did not get that, but that was what it was charged out at, so in real terms £500 to change a consumer unit is very cheap, and I would be wary, in 2006 I think it was we were quoted £100 extra, note extra not whole price to change whole fuse board for a new consumer unit rather than fit an isolator, henley block, and mini consumer unit for the bathroom, should have realised too cheap, the guy disappeared without finishing it.
So likely more around the £1000 mark for most houses, electricians quote on a win some loose some basis, or swings and roundabouts, and if yours only takes ½ day they win, if have to return next day they loose, and although they could do an EICR first, that also costs in time, so great in theory, but in practise often take a chance. And if using all RCBO protection should one circuit cause a problem all others still work, so can return next day to finish.
There is a lot of luck involved, my house had a borrowed neutral, luck traced to a 4 module light switch and only involved swapping two wires, others needed to use expensive kinetic light switches as found only two core and earth used for two way lighting instead of three core and earth. Clearly we can also have some luck where we have done houses in the same estate so have a very good idea of problems likely to find.
