Hot chocolate and cheese (Colombian Special)
Queso Blanco just means "White Cheese", which is kind of a catch-all term many locals use for simple cheeses. What is meant by Queso Blanco will likely change from region to region even within Columbia, and the chances of you being able to find it in the UK are pretty unlikely. Most imported Spanish cheeses tend to be the high-quality, specialist ones. 

You haven't put what the cheese actually tastes like in your post, so I am assuming it was most likely pretty mild and it is the texture of it that really floated your boat. When I think soft, mild cheese I usually think cows milk, not sheep or goat, although there may be non-cheese alternatives as well in the following list:


Panir may work for you, it's soft and will get softer, also pretty mild flavor
Tofu: ok, not cheese but texture-wise a soft tofu may work great, and the flavor will work with chocolate pretty well. 
UK white cheese: any major UK supermarket has a white cheese on sale, usually cheap, usually flavorless. Could be worth a try
Curd cheese: this is probably what would be called cottage cheese in the US. It's pretty liquid but if you drained it and compacted it you'd get a crumbly, easy-melting cheese


Let us know how you get on, I'm dying of curiosity!