Suze, once you get the electrical report there are a number of options. It's likely the valuer will give you a figure for current valuation once he's seen the reports. I've only ever seen a nil valuation in present condition (but a when repaired full figure) when a property has no bathroom or kitchen, not for electrics.
Most likely scenarios are
Option 1. Report goes back to the valuer, he says ok, if the electrical work is going to cost say £2000, and that's the only problem, then it's worth (say) £248,000 now and £250,000 once the works are completed. You then renegotiate with the sellers and show them the comment from the valuer that it's only worth £248,000 in its current state. You then buy the house for £248,000 and pay for the works yourself.
Option 2. As above, but sellers do the work before exchange. Valuer reinspects the property (small reinspection fee might be payable), gives it the ok, carry on as normal to exchange and completion. Valuer might not even want to reinspect, they might just want to see written evidence and receipts.
Option 3. As above, but you don't negotiate with the sellers. You pay full price, but the lender won't base your mortgage on the higher price, so you end up putting more money in and having to pay for the electrics as well. However, the lender may be able to release the extra to you once the work is done (this is called a retention)
Does that make sense