Can a UK citizen apply for a Thai tourist visa from Spain?

Yes it is

Before you leave UK, keep all documents in a file, ready to submit. Include your flights to Spain but 'forget' to submit an important document. File on the day of your flight to Spain. It will take about a week for them to email asking for the important document, stall for 3 more days and file on Friday afternoon.

It will take at least another week before they issue visa and you have bought more time. But they will want to see flights to Thailand and an onward flight.

You can book the Spain to Bangkok ticket and cancel within 24 hours for a full refund but it's usually a sweet spot booking 12 weeks in advance anyway.

Just go in visa exempt, get 30 days and extend another 30 while here.

Yes only if you have a Spanish residence,and you live there

Have to apply to the Embassy/Consulate in your own Country but as it is an online application you can do it from anywhere.

Or if you only plan to stay in Thailand up to 2 months, enter visa exempt for 30 days then extend 30 days

If you're not in Spain too long just apply in UK. Once issued it has a valid to use by date of up to 3 months. When you enter thailand, the visa is activated.

To use the evisa system you’d have to show proof of residency in Spain. You could try applying to the London embassy online but they’ll want to see flights. If you provide a flight to Spain and then a flight from Spain to Thailand you ‘might’ be ok but I’m not sure.