San Miguel is the same, beautiful in Spain, dreadful over here, even on draught.
It can only be one of two things, as you say the " Mythos " effect or the stuff that finds it`s way over here is somehow brewed differently .
I`d go for the latter, as one of the best pints of Guiness I`ve ever had in my life was from an Anglo Spanish bar in Fuerteventura, where the owner loved his Guinness and would only serve up a quality of beer that he expected to drink himself.
San Miguel that you drink in the UK is brewed in Northamppton like a lot of other beers to be honest: very few are actually brewed from their home countries.
Most the main foreign lagers now come from either there, Burton-on-Trent (the big one) or Manchester; very few are actually brought over from their home land.
That means the water content is different, which is very important as you'll see later, and the hops and barley are probably the same variety but a different source.
Guinness is either brewed in Dublin or Nigeria (they drink lots of the stuff), but it's produced in a concentrate form and then tankered to distribution plants.
The Guinness you drink in the UK is transferred from concentrate in Preston Brook, near Daresbury, so the major water source is difference hence the taste.
Your pint in Fuerteventura was probably made from a different factory and different water source, so will taste different; looking after it is important too.
I do still think the Mythos effect plays a part too: I brought Mythos back from a holiday in Cyprus where we'd drank it and Keo a lot and was lovely.
When I drank it in the garden however, it tasted like piss. I love Kronenbourg when in France, but the stuff they make over here is like cats piss.