not usually at an outlet store... maybe a bit ott for you, but a day trip to london is well worth it - the covent garden store has everything you'll need.
I believe the XL in flaks are the same as the rest of the XL models. They are just a larger size lens. Being better is really a preference for how you think they fit. For example, I like the XL in the m2s but not the flaks. So I have to concur, taking a trip for a day to try on various frames will save you stressing over a pair you don't really care for.
Oakley here in the states (not sure if it is in the UK) has a 90 return window. I have exchanged a few pairs now that after I wore them in the store and thought I liked them, I really grew to dislike them wearing them on a daily basis. For example, I can wear all of the PRIZM lenses except the daily prizms. Not sure why, but it's the only lens that gives me a headache. Once you find a pair you like in the store, get access to the exchange on here or find a reputable seller on eBay and get them cheaper. I loved the fuel cells and wanted a custom pair with vr28 BIP lenses. They would have cost me $180 at the vault. One of the members here had a pair up for sale for $80 with those lenses. And I got frames l liked better from him to boot.
Hi @BeccaBear sorry I'm so late to the party. I am in the UK too and definitely agree with @mjt42 Re a trip to London. Myself and @Herbsley went to Covent Garden last year and it was awesome. It's good to find a good store so you can try different styles. Shade Station in Leeds is really good too if you don't mind a drive. Wishing you luck with your first pair, if you've not already pulled the trigger. J x