Looks like it's 50 on Dual Carriageways, 60 on motorways[0]

But...speed limiters effectively limit all heavy vehicles to 56mph[1]

Note that speed limiter are mandatory above 3.5 tons, but the legal limits change at 7.5 tons.

[0]https://www.gov.uk/speed-limits [1]https://eptraining.co.uk/news/article/hgv-speed-restrictions...