This is to do with the ever changing stance on employees/workers and contractors by HMRC. It is not actually a straight forward issue although common.
Do you have an accountant as this is something they normally advise on.
There are ways that this doesn't actually impact you overall tax wise if the other companies are not doing the same but the fact that you use one of their workstations on its own no way merits them treating you as a worker. There are various other tests.
You can actually charge this time/cost against a Ltd company if you are also set up as a sole trader.
Sole trader charges Ltd Co for fees and sends invoice say £2000. Ltd Co claims this against it's tax (nothing to do with R & D - websites are perfectly allowable as an expense) and SAVES CT TAX AT 20%.
However you will have £2000 of income in your sole trader paid to you from the Ltd Co and you will PAY TAX at whatever rate you are at personally so the effect is generally TAX NEUTRAL.
If the website costs were external then they are perfectly allowable.
