The Eurostar ownership is not subject to franchising, and there is no proposal to repurchase the shares in Eurostar that were sold. There is no public policy in the Labour Party to re-nationalise any part of Eurostar.
This could change, particularly if Eurostar were to start having significant financial problems. Or if the quality of service were to drop below acceptable standards. However, there are only 10% of shares that are held by a UK stockholder. While the government could legislate to compel Hermes Infrastructure to sell back its shares, there is nothing it could do to require CDPQ to do so, as they are a Canadian company. A government could make an offer to buy the shares, but this would require them to pay over the odds. It would be very costly. Nationalisation in a globalised economy is hard to do.
As noted by Royal Canadian Bandit, Eurostar's rolling stock and other assets are located in the UK and subject to UK law. If it so wished, the UK government could expropriate them and give the owners whatever compensation it saw fit. The wider consequences for investment in the UK would be severe, so even a Corbyn-led Labour government is unlikely to do this
So unlike the regional rail companies, there is little chance of Eurostar being re-nationalised.