If you book a through ticket to St Pancras, it's only £4 extra above the ticket for Paddington from Melksham (open return, departing after 10am, with Senior railcard, done through
GWR▸ ). If you come in on platforms 7 or 8, there's ramp access to "first floor" level which gets you to gate-line access to the Hammersmith & City/Circle line, after which there are lifts to the platforms.
If you arrive at any other platform it's a longer schlep around the concourse, but you get to lifts to said "first floor level" on the concourse side of the platform 13-4 gate-line, thereafter to Hammersmith & City as above.
At St Pancras, if I recall correctly, two lifts give you step-free access from tube platform to International Departures, with helpful signs directing you to the correct lifts (I think E & F) to do so.
At Paddington, whichever route you take/have to take if you do not arrive on platforms 7 or 8, I'd guess it's quite a bit further than getting to the bus stop on Praed Street, but at St Pancras it must be much easier than getting off a bus in Euston Road.
Was this mentioned in the consultation, or
TfL» 's response to it? More to the point, have any forum members seen this route recommended or explained on any journeys they have taken via Paddington to St Pancras International? I only found out about the availability of "Euro High Saver returns" (Open returns valid for two months from nominated date for outward journey) from helpful Goring station staff, and worked the route out myself based on experience finding the new Hammersmith & City gate-lines after they were installed back in my working days when I had to travel up to London for meetings at venues best approached via that line.
Although I've done this route (using the tube) when travelling with my wife, who has some mobility problems not requiring any aids, I am not in a position to comment on how much worse overall it is than using a 205 bus. The fact it has not been mentioned so far in this thread implies that TfL did not mention it, nor investigate its suitability as a recommended alternative, which (if true) I find unimpressive for an integrated transport authority.
Of course, none of the above helps those seeking access between other London termini without the facilities at Paddington and St Pancras. TfL's emphasis on how much they sent communications to those resident along the route indicates an assumption that their buses are only used by locals, and that it did not occur to them that this service was also an important part of the cross-London connections for rail travellers on the national rail system. (On a route linking a number of termini stations, who'd have guessed?

. Perhaps us poor yokels outside the M25 don't count.