I am a big fan of cross country routes. This route was still running a year or two ago, even extending into South Wales, but seems to have stopped now. I think you have to change at Southampton. There used to be a train that ran a couple of times a day from Waterloo to Bath & Bristol, via Salisbury, I think. It was odd seeing a two coach DMU among all the 8 & 12 coach SWT EMUs.

I remember using the Cardiff service from St Denys (Southampton) but I think that may have only been from Portsmouth rather than Brighton.

I wonder whether the DMU to Waterloo was a way of retaining route knowledge 'just in case' for diversions e.g for accidents or engineering work?

I seem to vaguely recall a service from Manchester Piccadilly to London Waterloo, which ran via Bristol and Salisbury, with a two-car Class 158 DMU. Not sure how it got to Bristol, I suspect it ran via Hereford. Possibly it's part of the same train that Phil mentions.

I also remember Cromptons reaching Crewe (and possibly Manchester) back in the 80s.The service was from Cardiff, also via Hereford. I assume they had arrived at Cardiff via the cross country services that Phil refers to above.