HI Mandy

yes, club soda is soda water. Any fizzy non sugar drink will do - even sparkling mineral water. Alex liked the sparkling water as it had slightly less fizz which worked better for him. Soda water hurt his mouth whilst he had ulcers. Left to go a little flat and it was fine

Ensure is a brand of high nutritional food (liquid) and should be available in the UK. There are many brands and you may have heard of Jevity, or TwoCal. These products are usually available through your hospital dietician via the NHS. Supermarket brands such as Sustagen are available but are weaker versions and a poor second choice. They are supplements as opposed to complete nutrition. If you need complete nutrition (ie your diet is almost non-existant) you need to ask your dietician for recommendations and possibly to put you on a scheme that allows you to get it through the NHS (this is how it works in Australia - not sure about the UK).

Christine has answered your question re Magic Mouthwash - you need to ask your doctor about something to kill the pain of your ulcers that contains a topical anaesthetic to numb the pain in your mouth whilst you eat. That will get you to the UK equivalent