30. Onnec

Revenue: £129.7m (+28.1%)

Staff: 267

Onnec delivered strong growth in its most recent financial year on the back of an influx of business in IT infrastructure, datacentres, managed services and smart buildings.

It credited this spike at least in part to the "Covid hangover in 2020-2021 where projects were delayed."

The London-HQ business, formerly known as ExcelRedstone, also completed the acquisition of Swedish datacentre specialist Load Services AB.