Taxable profit and Corporation Tax

[Maslins]

With very obscure and limited exceptions, investments aren't tax deductible.

So you pay tax on £70k from your example.

However generally I wouldn't recommend any investing through a company; suffer the Higher Rate tax upfront, and invest personally. The long term CGT regime is better and your money is protected from trading risks.

This.

Although if you're happy for the investments to be held within a pension wrapper (meaning you can't access them until retirement) then that could be a tax efficient option, and would in effect lead to you just being taxed on the £50k (with the £20k being a pension contribution).