I know of one case recently who approached their situation as follows:
Bought house in 2007 for ~450k (had relocated to Ireland with work). Have since moved back to US. Put house on market for 360 k. House has recently gone sale agreed for 330k with banks consent. Vendor had a tracker mortgage (greater than 400k). Bank accepted the 330k as full and final settlement.
Vendor continued to pay mortgage even after they returned to US, so played ball with the banks and built a decent case that they had done everything possible to secure a purchaser at a very good price. If the bank hadn't agreed to the reduced selling price, then I'm sure next option was to hand back the keys.
Negotiation with the bank was done through a solicitor as far as I know. Most purchasers will run a mile from a sale that involves further negotiation with the bank, but if the price is fair and there's a chance of a good deal, then some buyers may hang in there (depends on the price and house I suppose).
In short, if I was you I would put the house on the market and see if you can get a buyer. Definitely worth trying to build a case where you tried to deal with the problem & the banks before just throwing the keys back. Will only stand to you if they ever do come after you.
Only my tuppence worth !!