What a great thread, the title does not really do it justice.
Well here is my two pennies worth.
UK vs Spain as a property investment market. No contest. UK wins hands down. Strong demand as demand exceeds current supply. Also, social changes drive demand as more married couples split up. Also, current younger generation more interested in live/enjoy now and sod later, so rental attractive proposition. Also in UK a mortgage market based on sound fundamentals (i.e mortgage cos will only lend what they can easily get back if there is default)
Location and local knowledge are essential. I have lived in my local town for 50 years, and know and understand my local market. It is booming now, but it is now our turn, I will fill my boots and then retire a rich and happy bunny, but it has taken many years for the boom times to arrive. However, I will not chase the boom times in other towns as I can’t touch and feel them.