I buy the Asda own brand Gaviscon, it used to be £3 for 500ml, it’s now £6.50!
I use to get Pinewood Healthcare Acidex Advance on presciption afew years ago. Just spoke to my brother and he got recomended Peptac by his mother in law. He buys several bottles which is often on offer to make postage worthwhile rather than paying for it on prescription.
It may be worth trying to see if its suitable – on offer for £3.85 at Express Chemist for 500ml ad £3.99 at Cheist 4u
https://www.expresschemist.co.uk/peptac-peppermint-liquid-500-ml.html