Where on the Uckfield line can you get dirt cheap Anytime PTE fares isn't Huddersfield-Leeds something like £3.5 for twenty miles?
Huddersfield to Leeds is 17 miles each way and is £5.50 Single and £8.70 Return (from January). This works out as roughly 33p per mile single. The 46 miles from Uckfield to London is currently £15.80 single, so is roughly 34p per mile. So the PTE fares are not "dirt cheap".
There have been a number of significant fare increases in the West Yorkshire PTE area in recent years, all on the grounds of having to pay for "increased capacity and better services", which most observers will say have not been realised adequately.
However, this is not just the busy bit.
It is not uncommon for people to have to stand all the way from Manchester to Edinburgh on Friday afternoons and all day at weekends. Similarly all TPE core routes are full and standing at off peak times such as weekends, and it is not uncommon for people to have to stand all the way from Manchester to York, Sheffield, Preston and beyond at many times of the day and week. During holiday times trains to the Coast (Scarborough etc) are often full and standing throughout from Manchester and regularly leave people behind at Leeds and York with the next train being in one hour. Fares for many of these journeys are not cheap. Manchester - Leeds (43 miles) is currently £19.50 single, £27.60 return. Uckfield (46 miles) is £15.80 and £26.20. Thus for a peak time journey from Manchester to Leeds where you have a high probability of having to stand the whole way or not even fit on the train in the first place you are paying 32p per mile, from Uckfield to London you are paying roughly 29p per mile for the same journey, with a much greater chance of a seat and not getting left behind.
So why does the Uckfield line need even more capacity and the TPE routes need LESS capacity to make the overcrowding worse. And the Uckfield line is only busy at Peak times in my experience. The TPE routes are busy all the time.