"The approximately £1.5Bn FSS contract would not be a problem for a major corporate entity like Navantia but Infrastra Plc is relatively small. "
Understatement of the year - Infrastrata was a tiny (2 or 3 people) onshore UK oil company that got into a gas storage scheme in N Ireland which mainly seemed to be about harvesting subsidies. They then took over the remains of Harland & Wolff, possibly smelling more Govt money. I'd be very very careful of entrusting them with any program of national importance.
One of teh real problems was identified by Parker in 2016 was that it's extremely risky to have one yard building two "first of types" - currently there are 3 major new types under construction the SSBN, Type 26 and Type 31e. Almost all of the construction experience and ability is already spoken for.