Mucinex - Which supermarkets sell this in uk?

The active ingredient in Mucinex is guaifenesin and is available in numerous branded preparations in the UK from both pharmacies and supermarkets. It is not available under the Mucinex brand.
Examples include:
Lloyds Pharmacy Expectorant Adult Cough syrup
Tixylix Chesty Cough
Robitussin Chesty Cough
Nirolex Chesty Cough
Boots Mucus Cough Relief
Guaifenesin is an expectorant and helps clear the chest by loosening and thinning secretions so that they can easily be coughed up. It also has a soothing effect on the throat.
It can be taken with most antibiotics but as you have not named the antibiotic you should check with your doctor or pharmacist first or you could check yourself using this link.
https://www.drugs.com/drug_interactions.html