It's rather disappointing that the Eurotunnel has coverage, even at its deepest depth of roughly 76m down, yet St Pancras International LL doesn't.
I'd imagine it'd not even take much cellular equipment to extend coverage to the platforms, though a leaky cable system through the core would be better.
Huawei were keen to provide it for LU, perhaps they'd be willing to do a trial installation on TL instead.
The coverage in Eurotunnel was rather expensive to provide. It was only available in one direction for a couple of years!