Robert wrote:

The machinery driving the 'boring plate'
would have to be scaled up to cope

One suspects that since tunnels are already routinely bored to the
larger dimensions on the mainland, such kit is readily available,
whereas the factory in Liliput making the UK-sized kit went out of
business ages ago through lack of orders.
--
http://gallery120232.fotopic.net/p9683684.html
(53103 (Class 116) at Lichfield City, 13 Jun 1985)