A road link was examined, but in the final reckoning was decided to be not financially viable. It would have been full of technical challenges, not least ensuring sufficient ventilation. The EuroRoute proposal (see the excellent article here:
https://www.roads.org.uk/articles/euroroute).
The Oresund is a 4km tunnel (10m deep) with 8km bridge, the transition happening near a natural island. The Straight of Dover is 30km wide and has a smooth profile from shore to shore, average depth of 46m. It is a completely different engineering challenge.
Dollands Moor is quite well used to be fair. The bigger problem was forcing HS1 to have some freight capacity, rather than focusing freight onto the classic network and leaving HS1 as a pure passenger line. Investment in the pinch points of the network would have benefitted regular passengers as well.