So if I set £3500-£4000 aside that should cover it right?
Good grief, whatever you do don't mention those sorts of figures to any sparks who come round to quote, or the job will suprisingly end up to be exactly that much.
First thing to do is establish what needs to be done, not how much you need to spend.
What I would do is lift some floorboards and look for old rubber cables. If you find any that means that your house has been rewired since it was built and does not need rewiring now. Personally if the wiring was PVC and in good condition I would not consider replacing it.
Ripping out good wiring is madness.
If I was you I'd do the following:
Get a PIR done for £150 or so, which will tell you what is wrong with your current setup, if anything.
And/or:
Change the CU for a modern one with RCD protection. This will involve a full test of all circuits too.
Get odd jobs done around the house like replacing sockets, adding some spurs here and there.
Total cost should be no more than £500.