If you look back at post #706
https://www.railforums.co.uk/threads/hs2-tbm-progress.240683/post-7642534 I did a quick back of the envelope based on the HS2 expected progress map:
"I just did some further back of the envelope based on the map in the pdf based on expected dates for the 2 intermediate locations:
Based on the map initial pace for the TBMs is slowish at 95m/week or 13.6m/day for the 1st ~2km, then 220m/week (31.4m/day matching the highest pace for short period to the west of OOC) over this summer for 1.75km of straight tunnel - this is is dead straight and the tunnels are widely horizontally spaced, then 76m/week (10.5m/day) for the last 45% with lots of curves, tunnels moving to above and below each other for the last ~1km and plenty of complexity (and presumably a long break at Christmas and New Year).
The average pace hides lots of variation..."
Mark W has since said the tunnelling window is 15-18 months and June 2027 completion was based on suitably pessimistic assumptions for 17 months. So if all goes well it could be April 2027 or not so well July 2027. They are also planning a long Christmas/New Year break which will also allow the segment deliveries to catch up.
Pace in the first 2 weeks is about a quarter of the average pace they reckon till the first marker on the map.