Back in 2010 the MoD kicked off the Military Afloat Reach & Sustainability (MARS) programme to plan a structured replacement of logistical support. Originally they planned on 11 vessels - 5 fleet tankers, 3 logistics vessels, two solid support ships and another tanker. Now it looks as if it might be the 4 Tides (all in service) and "up to" (and we know what THAT means) 3 solid support ships to repalce the "Forts".
It was reconfirmed in 2015 SDSR and initial assessment started in 2016 with invitation documents to design and build sent out Dec 2018 with "contract to be awarded in 2020" - almost certainly it will go abroad as they are going to be built (like the "Tides" ) to non military standards.