We were in almost exactly the same situation 5 years ago. The house was on at £410k, we initially offered £380k, then got it for £382k. The maximum we would have offered was £385k.
Did the full works similar to yours, rewiring, new boiler and radiators, whole house replastered and painted, new bathroom with four piece including double shower, new carpet on stairs and upstairs, plus put in downstairs cloakroom and wood flooring. The only room we didn't need to touch was the kitchen lol.
Spent just under £50k, and are putting it on the market this week for £465k. As long as we get an offer of £450k, we'll sell, as it means we'll come out with £150k equity.
If similar houses in good condition are selling for £400k, I'd offer £360k as a starting point, and go to £370k max. Don't underestimate how long completing works will take, or the cost. We offered in Feb of one year, completed in July, then moved in in Feb of the next year. We couldn't move into our house following completion, as DH project managed the whole thing, and we used separate trades. While we saved money by not bundling the whole job up into one, we spent a bit on continuing to rent and pay mortgage as we didn't give notice on our rental property for the first 3 months, then lived with family for the rest of the time. Also had a toddler and gave birth to DC2 during that time, so it was all a bit busy.
If I had my time again, I'd definitely do the same, and will most likely end up buying a property in a similar state, as can't afford a fully done house in the area I next want to move to!