At a green fair in Finsbury Park I came across this organisation which will install swift nest boxes in suitable sites. They were going to come last weekend and put nest boxes in one or two houses in this area but the weather was too bad for them to do it safely so we are waiting for another appointment . Although they are called Islington Swifts, the people who do the installation come from Cheshunt. The cost at the moment is £15 per box and £15 for installation.
Swifts are becoming endangered as they lose nest sites in older buildings like the Palace. Islington Swifts might be able to install boxes on other homes too, if they are suitable, they say that a two storey older house can be suitable. They can also give advice about doing building work which protects swifts, and are also interested if you know of current nesting sites as there is very little information available at the moment. You can contact them on email@example.com.
They have a website and a Twitter account (https://islingtonswifts.wordpress.com/ & @islingt_swifts).