In general, there are too many accelerators out there that don't do a great job. You might run the risk of getting bad advice or dead-end connections that could run your company into the ground. Or it might be great -- hard to tell without more information.
Sounds like financially speaking, you guys aren't in a bad situation if you're bootstrapping and being lean. Should at least be ramen profitable, which is a big success in itself. So you gotta now ask yourself how much more would an extra £19k do? Can you hire the people you want for how long you want them with that money, or do you need more?
Would this accelerator legitimately help you raise more, or can you do that without em?
I normally am against raising or applying to incubators without PMF. Too much unpredictability that can't usually be solved with more money -- just with more hustle. Then again, if running experiments, doing customer dev, or day-to-day operations require a large budget, then you might need the money simply to figure it out.