Coach from Cardiff back to Heathrow. Is the coach generally on time, so that we would be comfortable

Train from Heathrow to Llandudno Junction (via Euston). Given that our flight might be delayed, it doesn't make sense to buy tickets for a specific time in advance, but same day purchases are much more expensive. Can we buy tickets for a specific day but not a specific time?
Coach from Cardiff back to Heathrow. Is the coach generally on time, so that we would be comfortable scheduling our arrival at Heathrow 2-3 hours before our flight (rather than worrying we'll get delayed in traffic)? This would be on a Sunday, puerto rico car rental if that matters.
Interesting. You think it is too risky to buy a cheap, restricted train ticket to North Wales because of fear of flight delay. But you are comfortable with staying 150 miles away from the airport on the night before the return flight.
My risk profile would be totally opposite. I would be comfortable buying a train ticket out of Euston timed for about 4 hours after schedule arrival puerto rico car rental at LHR. But I'd be very uncomfortable staying in Cardiff the night before the flight home unless that flight was late in the afternoon and I could schedule the bus arrival about 4 hours before scheduled plane departure.
Return flight is at 5pm. A 9:30am train from Lladudno Junction should arrive at Heathrow at about 2:30pm (with a few transfers). I can't remember having a problem with fast-track security at Heathrow.
I guess my point is, if I miss my advance purchased train to Llandudno Junction, an off-peak puerto rico car rental ticket on a later train will cost me 78.30. If I miss my flight, a ticket on a plane the next day or on an inconvenient multi-stop journey would cost me $1500 to $2000.
puerto rico car rental OK -- there are NO trains from LHR to Wales, or to anywhere else besides Paddington station in London. puerto rico car rental And then you have to get from Paddington to Euston. You'll want to have 4.5 hours (or more) between the flight and train.