I think you might have to mostly suck this one up and move along.

If the police officers on the spot were sufficiently unmoved to take action, then your legal options are pretty limited...ultimately it would come down to your word against hers and you might well burn through a lot of time and money making your point

On the visa side of things, the EZ people you spoke with were probably (I'm sorry to say, and in the specific context of your interaction) in the right. After all, there are swingeing fines for carriers which allow boarding to international pax without proper documentation and staff are under strict instructions to follow the rules and tbh quoting Spanish law at UK check-in staff is almost the definition of wasted breath

Also, I suspect an EC261 claim for IDB will go nowhere as EZ will likely start and end with "Mr alexg32 could not demonstrate at time of check-in that he satisfied the visa criteria for the country he wished to travel to and he was therefore denied boarding..." (sorry if you're a Ms alexg32 btw - no way of telling !)

Nonetheless, I would still write a letter of complaint to Easyjet, setting out in a calm and objective manner:

1. The facts of the case
2. How you felt at each stage of the interactions
3. What you expect them to do about it

Their address for correspondence is:

easyJet Airline Company Limited
Customer Service
Hangar 89
London Luton Airport
Luton
Bedfordshire
LU2 9PF
United Kingdom

If it all went down exactly as you say, then you'll be helping manage a staff issue which could reflect very badly on the company (also yours may not be the only feedback received about a particular individual...)

Whatever you do, please come back and let us know the outcome !