I can imagine that it is only possible to buy those within the UK against proof of a commercial license.
There is lots of Nitroethane/Nitromethane sold on Ebay but many sellers ask for some form of proof.
This would be easy to fake tho I guess, you can try to find register numbers of companies that might have something to do with any use case of the chemical but do not have a web presence so that it is hard to find contacts. You can find those numbers on several websites, there must be nation specific websites for only UK companies but surely also EU specific websites for all registered companies in the EU. I'd try this only when ordering to an adress that can not be associated with me
Another option is Germany, Poland or China. There are many vendors in each of those countries that can ship those chemicals to you which might be willing to relabel them.
I have just shipped around 30 boxes containing 1x20L container each box with Methanol, Ethanol, Methylamine and similar liquid chemicals. I have simply labeled them as cleaning supplies and all of them have passed from Poland via Germany to Belgium.
As the transport of those goods in terms of logistical transfer from country to country is not illegal but rather just restricted this could give you another possibility.
You can do it the classic way and purchase it in a country like Poland from a company that will not ask you questions, get official export/import documents and ship it with an official courier..there are also lots of private couriers active on Ebay that drive weekly from Poland back to UK and vice versa, if you find one that still has capacities in his truck you could book the space quite cheap, they also ask less questions usually.
In this case I would use stolen or fake company data for the courier aswell as an adress that appears coprorate but can not be associated with me.
I hope this could help