Re: Type numbers?
It makes a bit more sense when you know they're grouped by type of ship and if a design study gets to the stage of being assigned a Type number it doesn't get reused even if it doesn't get to the build stage.
So Frigates are currently in the 2X series, having started at Type 11 which wasn't built, through the Type 12, also known as the Leander class etc.
We've had the T21 Amazon class, T22 Broadsword class, and T23 Duke class. The T24 and T25 were design studies which weren't built. We now have the T26 City class, well a bit of one so far.
Destroyers, Anti Air Warfare (AAW) platforms in the RN, started at T41 which went out of service decades ago, and were followed by the T42 Sheffield class. The T43 and T44 were design studies into a T42 replacement which ended up being the T45 D class.
The Type 31 seems to be using the 3X series to separate it from the Frigates as it's intended to be more general purpose than they are, Frigates being Anti-Submarine Warfare (ASW) platforms in the RN. I suspect this means they won't have towed array sonar. This is different to the Type 8X platforms which were general purpose in the sense they could do the full AAW and ASW roles, the most notable of which was the sole T82 HMS BRISTOL.