The main problem I found was the housing stock is in pretty terrible shape. Flats have basically no insulation and often have bad problems with damp. No insulation means the flats are freezing in winter and you can hear the neighbors do everything.
While eating out is definitely significantly cheaper, many things are (much) more expensive than in London. Supermarkets, home internet, cell service (brutally expensive), utilities (I was paying 10x for utilities in winter compared to London, because of the complete lack of insulation - and Portugal has some of the highest electricity rates in Europe).
Overall I would not suggest moving to Lisbon if you are expecting an overall significantly lower cost of living than London. There are lots of other benefits though :).