Hi Kiwi
I'm currently doing look-ups at the London Metropolitan Archives and Westminster City Archives where the registers have been deposited.
I'm happy to add you to my look-up list and check these dates and places out for you on a future visit.
Having looked at the Pallots image, I am guessing that the 18 might relate to a register page number rather than an age as I would not expect to see an age in a register from 1810, the term 'minor' would have been more frequently used.
Their marriage licence was issued by 'Marriage Licence Allegations - Faculty Office', a copy of this should be at the Society of Genealogists in London (sorry I'm not a member) or through the Origins Network site, but this is a 'pay to view' record.
Dawn