I was travelling from Westcombe Park to Paddington once (single
required, I was catching the sleeper to Penzance) and used the ticket
machine at WP station (ticket office closed).

I duly typed in my destination as Paddington but it only charged me the
normal Paddington to London Terminals fare (£2.50 if memory serves).
The ticket recorded the destination as London Terminals but I still
hoped that I'd be able to get all the way to Paddington for this price.

Probably needless to say, when I tried to use the ticket to enter the
Bakerloo line at Charing X, it was rejected. I ended up having to pay
£3 for a single to Paddington.

I was a bit miffed because I felt that if I'd been sold the correct
ticket at WP station, it would have been less than £5.50 (=£2.50 +
£3).

Or is there a round-the-houses route from Westcombe Park to Paddington
on surface rail only (ie avoiding the underground) that I could have
used my ticket for?

Regards