I think the OP may be being unreasonably deterred from a UK route by some of the (very knowledgeable) rickshaw/tricycle experts above. There are some good routes which would not require any climbing more than (say) 7% and that only occasionally. For example from south Cambridgeshire to Glasgow via Spalding, Lincoln, Humber Bridge, York, Teesdale and Moffat. I have in mind the 100-630 section of LEL (more even than @Pale Rider has suggested - the climb up and over Yad Moss is long but I don't recall any steep bits.) Perhaps it's time for the OP to see what sort of climbs he can reasonably get up with the current gearing (as part of a long day's ride) and then construct a route avoiding English main A roads accordingly - happy to help with the latter btw (if UK). I would be tempted to look at a 'protection for passengers from the wind' arrangement (not a modification) which might significantly reduce your drag.