The lower part of the address is crossed out but appears to have been addressed to try Bath first before going to London (one can just about make out "Bath, England" at the bottom). It has a red framed "INDIA LETTER / PORTSMOUTH" - as an India letter it had a reduced ship letter charge of 4d for up to 3 oz.
The double framed "RETURN'D / FROM BATH" cachet is used two years later than previously known examples.
The entire has a number of postal markings, most of which are crossed out ! I believe the first marking was the "2/7" which would have been 4d India ship letter charge plus 2/3d for the triple letter rate for the approximately 95 miles from Portsmotuh to Bath. I think this was then inspected and crossed out, being replaced by "3/4" with initials, which would have been 4d India ship letter charge plus 3/- for the 1oz inland letter rate from Portsmouth to Bath.
I believe this was paid in Bath and it then got charged a further 3/- for the 1oz inland letter rate from Bath to London. The "6/4" crossed out could have been a charge for the whole route Portsmouth - Bath - London which it seems was cancelled presumably because payment had been made in Bath.