A righrs issue is an offer to you as a shareholder from the company, to purchase newly issued shares at a discount from market value.
You can either take up the offer and purchase the shares, or sell your "rights" to purchase your shares.
I imagine that you sold your rights (or did nothing, and so in effect sold them), and this is what you have received the cash for.
If the above is the case, you have made a Capital Gain on this part disposal of shares, and should prepare a CGT calculation tkaing into account the history of your original shares to calculate the gain on the part disposal.
If the amount is minimal, and under the annual exemption there will be no Capital Gains tax.