I can think of a number of routes that routinely reverse once:
- Nottingham - Crewe at Derby
- Nottingham - Skegness at Grantham
- Maidenhead - Marlow at Bourne End
- Middlesbrough - Whitby at Battersby
- Liskeard - Looe at Coombe Junction
- Plymouth - Gunnislake at Bere Alston
- Heart of Wales line at Llanelli
- Inverness - Wick at Georgemas Junction
- Glasgow - Mallaig at Fort William
- Lancaster - Heysham Port at Morecambe
- Cross Country services at Reading
- Cambrian Coast services at Shrewsbury
- Holyhead - Cardiff at Chester
- A number of Manchester Airport services at Piccadilly
- Caledonian Sleeper Highlander at Edinburgh (or Wembley when diverted via ECML)
- Caledonian Sleeper Edinburgh - Euston at Carstairs (or Wembley when diverted via ECML)
- Great Central Railway Nottingham reverses at Manor Park
Milford Haven to Manchester Piccadilly services reverse at both Carmarthen and Swansea.
The 0518 (Mon-Sat) Lincoln to Nottingham calls at Newark North Gate, reversing there and again at Newark Crossing East Junction.
The 0639 Mon-Fri / 0642 Sat Nottingham to Reading XC reverses at Derby and Birmingham New Street.
Way back in the day there was a weekly Summer Friday overnight HST service from the North East to Cornwall via Paignton, reversing there and Newton Abbott. I can't recall whether it went via Gloucester to add a third reversal.
The Royal Scotsman has a leg from Kyle of Lochalsh to Boat of Garten with reversals at Welshes Bridge (Inverness), Inverness and Aviemore, however I don't think that was the kind of candidate you were seeking.