I’ve always been nervous about driving on the wrong side of the road over there (or maybe it’s us that drives on the wrong side!). Even their rural trains run like streetcars, and the local bus services are great, altho agreed it involves a little more planning and it’s much more convenient to have your own car. And we’ve also used holiday “lets ” as they’re called, altho lots of B&B’s plus small hotels like the Royal Sportsman in Porthmadog.
For non-railfan interest, the castles are both ancient and amazing, so don’t miss Harlech Castle on the Brit mainline, accessible by train between Porthmadog and the Talyllyn Rly at Tywyn. Also Caernarfon Castle, a short walk right at the end of the Welsh Highland Rly. Or Conwy Castle perhaps on your way to or from Manchester - the Brit railway mainline goes right through the castle walls.
And if you like beer, beef and lamb, you’ll be sure to gain a few pounds. The weather will be nice and warm by that time of year, and it won’t be tourist season yet. Whether you get rain or not is luck of the draw, but dress for it and enjoy the train rides!
Cheers,