Several points here - if you are
going to be buying and selling land and property
, wherever situated, it is likely to be considered a trade, hence subject to income tax (and Class 4 national insurance) rather than CGT. Have you checked which tax you would actually be subject to in Poland?
Double taxation treaties allow relief for tax paid abroad on income or gains taxable in the UK, but if you paid CGT in Poland rather than income tax you would probably not be able to offset one against the other. CGT is calculated on gains, which are generally calculated in a different way from profit.
If you are operating as a partnership, the profit is declared on a partnership tax return, but there is no tax calculation on the partnership return - each partner's tax is calculated separately, and any double tax relief due would be claimed on the individual's tax return, on the Foreign pages.