https://www.tesla.com/support/energy/virtual-power-plant/pue...
You need to buy a battery from a vendor who is orchestrating a virtual power plant program, and live in an eligible geography.
All that's really required is a suitable power meter that links up with a central control authority to orchestrate the whole thing. That's where the complexity lies.
There are hundreds of thousands of residential batteries in the UK already, and ~0 of them currently can be called on for demand frequency response. That's because 'demand response' is not something you can typically be credited for on your electricity bill. The closest you get is you might have a half-hourly varying pricing scheme encouraging you to discharge your battery when prices are low. The only UK supplier to do that so far (octopus) still uses day-ahead prices though - so you aren't reacting to market conditions, but instead predicted market conditions 24 hours ahead.
I think the fix for the uk is for the regulator to step in and switch half hourly pricing for second by second pricing. Get rid of 'frequency services' (unnecessary with second by second pricing). Then make most/all users of the grid pay those second by second prices (no more 'fixed X per kwh' contracts).
All electricity contracts should be pay-in-advance, and meters should have an option to cap spend per hour/day.