If I pay for 10kWh from Green Energy Co, they source it from renewables and put 10kWh into the grid. You might think they don't, but they do, unless you can show they are doing shady deals with non-renewables to provide that energy?
This is the problem, if renewable sources are available they will be used irrespective to you buying a "renewable" only tariff. If you are supplied by the national grid your effectively using the same proportion of renewable and non-renewable energy as everyone else on the grid, or at least in your grid network area.
Maybe you can say you contribute a very small amount of extra funding (aka the retailers cut which was around 5% before the price spike) to a renewable generator as Green Energy. I believe actually generate some of the energy they sell?
So you could say you are very slightly helping to fund expansion of that. But to be fair, if it's profitable to make renewable energy for the national grid your contribution to fund that expansion is a tiny proportion of the investment used.
If they had no retail customers they'd still sell all the energy they generate to the grid at the same price and all the energy from any other renewable source they have purchased from would also still be used. (until a time when renewables out supply usage)
Buying a renewable tariff is a nice marketing strategy to make customers feel better.
Non-renewable are only used when renewables can't meet the required capacity and load balancing.
As an example, right now at 25.7 GW demand.
52.5% renewable energy (Solar, Wind, Hydro)
25.4% other energy (Nuclear, Bio, pumped)
11.6% interconnectors (Mixed sources, Nuclear and wind a significant part)
10.5% fossil fuels
If demand drops it's fossil fuel generation that is reduced first where possible. If every Green Energy customer decided to have a no power usage day today, it would be gas power generation that would be reduced on the grid, not the renewable energy you think you have purchased.