When a company issues new shares, the money it received is split into two conpoments - one is the amount of the share itself and the other is a premium depending on the amount paid
Aviva shares have a nominal value of 32 17/19 pence
The share premium can be returned to investors which is what Aviva is doing.
The effect on you is to reduce the cost of the shares, so if you purchased the shares at £ 300 each, then your purchased price is now reduced by £ 1 if I read the announcement correctly
No tax is paid - it will increase any CGT you have to pay when you sell the shares