Can anyone tell me what is the largest freewheel cog that I could fit to my Brompton H6?

My Brompton H6L has the BWR Hub and two speed dérailleur.

So gearing is 50T front and 16/13 rear which with 16" Schwalbe gives:

32.5/40/50.9/62.7/79.8/98.2 inches

I need the range to be lower and higher

A 53T front with the same rear would give me 35.1 to 106 inches but not low enough

A 53T front with a 20/13 rear would give me 27.6 and 104.1 inches

This would suit me fine but I guess that 20/13 is an impossible dérailleur change.

So looks like a Schlumpf Mountain Drive with the standard 52T/16/13 is the solution?

13.6 to 103.7 inches

Ian