The use of the word "mall" to mean a shopping centre has come to BrE from AmE. I had never heard anyone in the UK say "shopping mall" until about ten years ago. I have heard people pronounce is so it rhymes with "fall" and others so that it rhymes with "gal". If there were any logic to British pronunciation, it would always be pronounced to rhyme with "fall" because every other one-syllable word ending "fall" is pronounced the same way (all, ball, call, fall, gall, hall, tall, stall, wall). However, it's not a BrE word. I think it's likely that people who have heard an American say the word aloud probably copy it and rhyme it with "gal/Sal/Al", and those who have never heard it said aloud assume that it follows the pattern I mentioned above and say it so it rhymes with "fall".