You seem to be trying to pretend that an overstay is no big deal.
It's a big deal. Fix it.
Someone has identified an extremely affordable flight, and you need to be on it so you are in UK soil by midnight on the 17th.
You created a belief in your mind that Spain is "relaxed" about overstays. You are missing the entire point of what an overstay is. Perhaps you think the consequences for the overstay occur at exit, and they make you pay a $500 fine or if you only have $40 just settle for that, or whatever, and the thing's forgotten. That is wishful thinking.
In reality, if you overstay, they will cheerfully let you leave Spain, no problem. (Since that is, after all, what they want you to do: Leave). The consequences will come later. When you are trying to enter another country, or revisit the EU again. The countries will observe that you have an overstay. If they interchange data the overstay will flag up on your record along with the officer's notes when you interivewed in June (if your story at that time doesn't match up with your 100ish day stay, they'll know it).
This will result in refusal at the entry. You'll have paid round trip airfare, hotel, concerts, whatever - you'll arrive, get refused, and be forced to pay full boat list price for a flight home. That happens pretty quick if you flew from the USA, but for example you'll be flying from Spain to the UK. The UK can toss you in jail until a flight to USA can be arranged.
Then if you want to fly in the future, you'd need a visa because of the refusal. Visas are expensive.
If you're a YOLO kind of guy who doesn't think about the future, no worries, it's all cool, man. But if you care about your money and travel rights, it is very, very stupid to overstay simply to score a cheaper airfare. The down-the-road costs will make it the most expensive airfare in history.
Another poster confirms last minute bargain flights exist. Be on one.
Life will be much easier down the road if you take care of this today.