Interesting… No one has an idea or no one else trades on multiple accounts?
I think it needs to be averaged from all platforms. The reason is very simple.
This is from HMRC website:
"## How to identify the shares disposed of
When you dispose of shares you cannot work out your capital gain or loss until you have matched the shares disposed of with shares you acquired. You’re treated as disposing of shares in the following order:
| Description | |
|---|---|
| First | Shares acquired on the same day as the disposal (the ‘same day’ rule). |
| Second | Shares acquired in the 30 days following the day of disposal (the ‘bed and breakfasting’ rule) provided the person making the disposal was resident in the UK at the time of the acquisition. |
| Third | Shares in the Section 104 holding." |
So if shares are not averaged, one could sell, transfer funds to other account and buy shares somewhere else. I guess the “same day” would not work with different brokers, since there is no broker who would transfer money after stock sale the same day (is there one?) But it is possible to get your money from one broker and transfer to another broker in less than 30 days. So bed and breakfast rule would be not valid.
Am I wrong on this? If not, I guess it is better not to hold and trade one and the same stock across multiple brokers, since it would create nightmare when averaging the price.