no one has actually sold their shares, they have simply...

no one has actually sold their shares, they have simply transferred from a standard trading account to a tax protected account, to achieve that a "sale" and a "buy" are involved. Positive take is that the transfer has been made to avoid tax in the event of a big increase in SP.

See wiki 88 for more details;

The Benefits of an ISA.
https://www.moneyadviceservice.org.uk/en/articles/stocks-and-shares-isas

Bed & ISA
Bed and ISA transactions allow you to sell existing investments and use the proceeds to open or top up an ISA account. You can then buy the same investments back, choose other investments or simply hold the cash within your ISA.

"Capital Gains Tax with a Bed and ISA
When you sell your investments to begin your Bed and ISA transaction you may have to pay Capital Gains Tax if your gains for the current year exceed the annual allowance (£11,700 for 2018/19). If you make a loss, this could offset any other capital gains you have made this year or in the future."
https://www.bestinvest.co.uk/stocks-and-shares-isa/bed-and-isa