Impact on Bondora returns - UK tax rules

That correct.  Companies are presumed to be undertaking a trade, in this case it would be one of money lending/trading of debt, and as such all gains/profits will be taxed as income (in general corporation tax does not distinguish between "capital gains" and trading income and both are taxed in the same way.

However, you don't have to be a company to be treated as conducting a trade.  If your pattern of behaviour can indicate that you are undertaking a trade versus casual transactions.  Eg, if you frequently buy/sell loan parts in a systematic way in bulk, these are all strong indications of undertaking a trade rather than making investments.  A useful HMRC guidance on badges of trade can be found here: www.hmrc.gov.uk/manuals/bimmanual/bim20205.htm.  The tax implications if you are deem to be undertaking a trade in money lending/debt are significant.  All gains/income from the activity would then be subject to income tax, including 'capital gains' and platform incentives/cash back (which would have been otherwise outside the scope if they were paid by the platform rather than the borrower).

People in the past have been caught out by failing to understand that they were trading and should thus should have been subject to tax on income.  Classic example are gambling and spread betting which are not normally subject to income tax, but are if your transactions are frequent, systematic, have a profit motive and tick other 'badges of trade'.  That said, the normal presumption is that most people will not be undertaking a trade and isn't an issue unless HMRC believe that they are suffering a significant loss in tax take.