This is to do with the employment related securities rules. These are very widely drawn up and catch the majority of employee situations when staff get shares (the only time it doesn't apply tend to be where the award is clearly as a result of family transactions rather than employment relationships). Many people assume 'Founder Shares' don't fall under these rules, but unfortunately HMRC believe that even these are caught.
The issue is around the value you paid for them on day 1. If you were provided with the pref shares at a discount then this discount is taxable on you at the time you are given the shares.
So if you bought 100 shares for £0.01 (£1) and they were really worth £10 each (£1,000) then you've had a discount of £999 which is immediately taxable on you as income (on purchase - not sale).
If you later sell them for £20 each the gain from market value at acquisition (£1,000) to sale is taxable under CGT rules - so £1,000 * 20% = £200. However, bear in mind you've probably got £400 of unpaid income tax from the original issue.
Obviously if they were worth 1p at the time of initial purchase and that increased to £10 for the later purchase there's no issues, but with round figures like this it's unlikely to be accurate valuations.
The other thing to consider, after all the above, is that the payment in relation to the preference share may have rolled up dividends which would be taxable as income in any event.
I'd suggest that you (or your employer) takes some professional advice on the subject.