Best way to get from Prestatyn to London and then to LHR

Skip trainline and use https://www.avantiwestcoast.co.uk instead, the company that actually runs the trains from Prestatyn to London. Although they take you to Euston and not Paddington.

There are three possible routes-
1) The Holyhead to London Euston direct trains run by Avanti West Coast (or Transport for Wales local services to Crewe, for Avanti connections or London North Western to Euston),
or
2) The 2 hourly Holyhead to Birmingham trains run by Transport for Wales, for Avanti, London North Western or Chiltern connections to Euston (Chiltern go to Marylebone),
or
3) The 2 hourly Holyhead to Cardiff trains (on the other hour to the Birmingham trains) run by Transport for Wales, for Great Western connections to Paddington, changing at either Hereford or Newport

There aren't through tickets to Paddington via this route- has to be booked in 2 stages. Thus the system will be trying to send you to Euston, with a tube connection to Paddington. Avanti have not yet released tickets for May, and are not scheduled to do so until AT LEAST 22 March, maybe later.
Also note-
a) Transport for Wales have not yet released their Advance Tickets for Sundays in May,
b) Due to engineering work further down the line on 7 May, all services from Prestatyn will be modified on that day- timetables not yet available.
In short you are too early by at least 3 weeks, maybe more.

in addition to the above, the gang at Avanti aren't too swift in honouring their timetables when they do release them.

Are you sure that you will walk all the way from Chepstow to Prestatyn? What if you get half way and have had enough? If that is the case, note how you could proceed. In the Knighton area you come across the Heart of Wales Railway (1 train very 4 fours though) which could take you to Shrewsbury from where you could either head north to Chester, east to London via a change at Birmingham and then to London Euston our go south (change at Craven Arms) down to Hereford (or even Newport) and then a train to London Paddington.

Going north on Offa's Dyke path, the next railway you come across will be in the Welshpool area with trains to Shrewsbury.

If you pre-book a specific train with an advanced ticket from Prestatyn/Chester and don’t make it - you lose your money. Even booking a day or 2 beforehand can save money compared to turn up and go for longer journeys.

Thanks, all, I will look into your recommendations!

James, yes, we are planning to walk all 177 miles from Chepstow to Prestatyn. :) I've done village to village walking trips before and it's my favorite way to travel! We have accommodations booked along the way and a company to move our luggage from inn to inn so only need to carry a day pack. And, if any of us DO need a day off, we can always grab a cab to the next stop.

Hi katkatf -

I really enjoyed doing Offa’s Dyke Path some years back, so if you have half as good a time as we did, you’ll have a blast! There was only one ‘dodgy’ accommodation we stayed at where, after being outside all day, we found it warmer to hang about outside than stay in our room. Then we headed for the pub as soon as it opened! We also walked it north to south because in her infinite wisdom Julie decided that “we’ll be going downhill”. She was soon disabused of that notion!

Have fun!

Ian

Hope you plan to divert off the path and visit Bishops Castle

Thanks, Ian. That is funny about North to South being downhill! I have done several other paths, and really love it as a way to see a country. I am excited about Offa's Dyke because it has some history from different eras as well as looks just plain beautiful.

Kathy