I'm not aware of the actual situation. But most buses have place for big luggage. Or in the trunk of a coach/intercity like bus, or some polyvalent space if it's more of a city bus. 

Nothing to worry about I reckon. 

I really hope so, otherwise it will be such an inconvenient, thanks for the information!

I would expect a mix of coaches with proper luggage storage underneath and buses with very limited luggage space.

Unfortunately there are now rules on accessibility that are pushing the use of (really unsuitable) buses over coaches so good luggage space may not be guaranteed.

Alternative routes that avoid replacement buses are available:

Crosscountry trains Newcastle - Sheffield then East Midlands Trains Sheffield - London St Pancras would be the fastest.

A little slower but possibly less busy would be Northern Trains Newcastle - Carlisle then Avanti Carlisle - London Euston (there are 2 routes Avanti takes, avoid trains going through Birmingham they are much slower than the direct trains with less stops).

If you post up your travel times I’ll look at the best options.

But the route from Sheffield to London St Pancras is going as usual? I supposed there is some engineering work ongoing?

The route via Sheffield is running as usual, it’s a detour : Newcastle - London trains generally use the East Coast Mainline (via Peterborough). This one is closed after Peterborough.

Expect trains on this route to be very very busy though. I’m not sure what’d be better honestly.

Appreciate that! Thanks