The Island Cafe in London

The Island is a greasy spoon with wifi, and it's brilliant at both of those things.

The wifi has always been strong and reliable on the days I've spent doing work there, which was while others were connected too.
The food is standard fare, sandwiches, salads etc. Perfectly decent, and cheap too. The staff are pleasant, fast and helpful.

For sunnier days the outside area is great, and yes, the wifi works out there too. I keep going back and I don't intend to stop anytime soon!