Will I have issues entering the UK from Spain as a Canadian due to covid?-

I recently purchased round trip tickets from Canada to Spain stopping in London. I made the purchase through kiwi.com (absolutely never use this site) so the tickets were separate. I was able to fly into London with no problem, but when checking in to my flight to Spain, the Ryanair person told me I wasn’t able to board due to Covid regulations. I checked the EU regulations, and Canadians are allowed to enter Spain. However the Ryanair person told me that because I was flying through the UK I wasn’t allowed to board. That same day I tried purchasing a flight to Portugal in person with an easy jet representative, but I was told that I wouldn’t be able to board either. I ended up traveling into France via train. Can someone please explain why I wasn’t allowed into Europe by plane from the UK?

My next, and more important, question is if I will be allowed into the UK by plane from Spain? My flight to Canada leaves from London. I am currently in Spain and need to fly over to London to catch that flight.