In the SE where I used to work it surprised me that the North Downs Line between Reading and as it was then Tonbridge escaped the axe, yet two lines which would have been highly useful now between Shoreham-By-Sea & Christ's Hospital via Bramber and Southwater and the Christ's Hospital to Guildford line, they used to be linked via a triangle at Christ's Hospital.
I'm sure that it would be a useful route for trains to/from Brighton to not only London, but the Midlands & the North too.
As for down here in Somerset, the line from Langport to Yeovil for Axminster/Salisbury would be quite busy now as it's a convoluted route to get there by rail, it's either down to Exeter & up or up to Westbury & down to Yeovil via the infrequent Weymouth service or at times it's even up to Brizzol and back across to Salisbury.
The Somerset & Dorset was another great route, that was lost to the idiot Beeching, it breaks my heart to see it gradually returning to nature; I'd love to see it return, however its been built on so much it would I think be impossible to return it to use.