I have dismantled one of these devices (don’t ask why), as scotman says, they contain only capacitors and any energy savings will be minimal (<0.005%). As to whether they will lead to a power factor correction will depend on the electrical load (within your home). But since domestic consumers in the UK are billed per kWh (i.e. per watts used) and not volt amperes – by improving the power factor of your installation will help the power company, but not reduce your bill.
The claims made for these devices of double digit percentage savings are complete nonsense.