The recipe looks to use a larger mug than the 8oz-or-so that many UK mugs hold. Some of the ingredients will shrink when wet meets dry, but in pre-mixing volume terms there is about 12 fl oz there.
Australian and "old-school-imperial-measures" users will most likely have a 20ml tablespoon, but I suspect this uses the 15ml which has become the norm in most of Europe an N America.

I would suggest that experiments start with something like a pyrex mixing jug.   I haven't tried this recipe, but can confirm that sponge cakes will work in a microwave, though they look slightly odd to me due to a lack of browning on the outside.

I do know one lady whose guilty secret indulgence is to make herself an individual treacle sponge using microwave and mug. Keeping all the ingredients together in the cupboard, she claims to be able to make one, eat it, and wash away the evidence inside 10 minutes. I will see if I can wheedle the formula out of her.