OP, have you had a drains test done? Drains can cause a LOT of inconvenience and expense if they are damaged and need to be replaced.
Have you run all of the taps in the property, and flushed the toilets? If not, go back and do this, so that you know what you are getting into.
As an ex estate agent, I am appalled that you weren't told about the dodgy electrics. However, you do sound a tad naive, is this the first time you've done a renovation? My advice to you, would be to go through the survey you've had done with a highlighter, and mark everything which has been pointed out as being a problem. Then, get builders in to quote for the necessary works. If the property has been priced with a view to the work that may need doing, then take that into consideration, ie, if the property fully done up would normally be marketed in your area for £500k, but due to the obvious work needed, it's being marketed at £450k, then they've already made a good allowance. However, as you were led to believe the the electrics were OK, then definitely ask for the whole cost of rewiring.
Are you an experienced DIY'er? If not, be prepared that this house is going to cost you a LOT of money to get into good shape by the sound of things, and it doesn't sound like you have a massive budget. Has the survey down valued the property at all? If so, then you need to ask for the price to be reduced to the valuation given by the surveyor.
So basically, do your homework, and then send the agent an email with a revised offer, stating why it has been revised, ie, we've revised our offer based on the fact that we've now had a survey done, and it has brought to our attention that the property needs a full rewire, cost of which will be ..., etc., etc., and then be prepared to walk away.
Finally, being NICE when buying and selling a house is foolish, does your partner really think that the Vendor will be nice when reviewing your amended offer? Of course not, they will want every penny they can get, and in turn, every penny you save on the purchase will be needed to put toward the renovations, so tell him to stop being nice, and start thinking with his wallet!