Hello,
Unfortunately no really easy cheap way around this- the TfL PAYG Oyster is only valid to Potters Bar on the King`s Cross- Hatfield- Welwyn Garden City -Stevenage Line, Borehamwood and Radlett on the St. Pancras- St. Albans-Luton-Bedford line. Hadley Wood & Borehamwood are where London zone 6 finishes : from there on National Rail fares apply. There have been moves over the years to extend the TfL/Oyster zone 6 to Potters Bar/Welwyn Garden City and St Albans/Luton but so far without avail. Normal contactless payment is valid at Hatfield and St. Albans however, and for a single fare in my experience normally works out cheaper than a paper ticket.
TfL PAYG Oyster has been extended to Hertford East and North stations.
Regarding the TfL `alternative routes` If my memory is correct, this means by travelling via/through these stations so you avoid crossing into zone 1 and therefore the extra cost- I did look at this when going from Hatfield to Romford recently, but in the end as I wanted to visit friends in Hackney as well bought a soon to be ended (thanks a million Mr. Khan) one-day Travelcard, discounted with a railcard.
UNO run the 614 from Queensbury Underground to Hatfield Station and return, with a peak hour 644 variation via Borehamwood then straight up the A1 to the UH campuses whereas the 614 travels via Barnet The Spires rejoining the A1 at the M25 South Mimms interchange.
If coming from East London an alternative could be the Underground to Cockfosters, then the UNO 610 which has been extended from Potters Bar ( but subject to delays.) Another alternative but a bit protracted is to Hertford North or East, then the 724 Harlow-Heathrow Greenline limited stop bus from there.The UNO 341/641 travels via Hertford but not directly, all buses serving the dH campus but only some the College Lane one.
Regarding the fares, the Government sponsored £2 single bus journey outside London still applies until the end of Oct, thence on it will be £2.50p until the end of 2024. I believe, but will check it out, this also applies on the 614/44 from Queensbury (Oyster cards except Zip cards are no longer).
Incidentally, for those travelling from Luton, Bedfordshire Council have agreed after the 50p rise in Nov to continue to underwrite the extra 50p on the £2 fare, while Hertfordshire will be at £2.50p I`ve been told today..So from Nov 1st. Luton to the UH at £2, back at £2.50p.
Unfortunately the limited stop Green Line 716 bus, which UNO took on for a while, that ran from Victoria to Stevenage calling at Finchley Rd. and the two UH campuses was stopped a few years ago as part of the cuts.
This is the Hertfordshire County Council`s own travel site you may find of use
https://www.intalink.org.uk https://www.unobus.info is the UNO site.
Hope this helps, don`t hesitate to ask if you have any more queries .
Finally, I think this still applies (the UH folk reading this can correct me if wrong) is that if you bank with Santander while a student ( caveats attached of course!) they will give you a free student/under 25 railcard for four years which will be a great help in reducing the costs.
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