Gosh hard to tell if it will work or not without knowing Ipswich. It might work better in a 'tourist town'. Is Ipswich a tourist town?
If it opens, I would go if/when I am next able to get there to go ancestor hunting
Googling, there are a few such places in the UK at the moment, but their menues don't look terribly authentically 1940s. I suspect an inevitable morph into things that suit more modern expectations is almost inevitable, if money is to be made. They have opened various English teashops near our house in France, over the years, and the only one that's survived is the one that began to include French meals on it's menu i.e. people want what they can normally have. But that's France (or rural Normandy) for you, perhaps.
The Home Front tearoom in Ramsgate sound more '40s than some other such establishments:
"We serve sandwiches, including spam, corned beef and other popular fillings, jacket potatoes and items on toast, pilchards being our best seller. Our tasty treats include jelly, fruit cocktail or peaches with either ice cream, custard or evaporated milk. We also serve scones, crumpets and teacakes, 17 different teas, various coffees including camp coffee, Ovaltine and Bovril. Our range of cold drinks include Lemon Barley water, Vimto cordial, Tizer and lashings of Ginger Beer."
But... I am not saying that this place embodies this, but I think most people don't really want the past served up in an authentic form. They want a contemporary version of the past. A pastiche. Fabrics in the 70s were not, I think, as swirly, and certainly not as colourful, as young people going to 70s parties think they were. etc.
“In the midst of winter, I found there was, within me, an invincible summer." Albert Camus