What time of year are you visiting?
Is "Reading.. Folkestone....Wales....Snowdonia" your predetermined itinerary? Snowdonia is in Wales, so whereabouts else in Wales.
The standard answer if you are after just numbers of species of UK birds is North Norfolk, but that is not that near "Reading.. Folkestone....Wales....Snowdonia".
What time of year are you visiting?
Hi Garuva. Welcome to BirdForum. If you're in Bridgend I'd recommend a visit to Penclacwydd. C40 miles West.
Rich
Near to reading, otmoor and farmoor are worth a visit can be good daily sighting usually on http://oxonbirding.blogspot.co.uk/, you can often get close views on the causeway at farmoor of waders and wagtails and otmoor can get good views of Red kites and smaller birds on the feeders
If going from Bridgend to Snowdonia up the A470, shortest but not necessarily quickest route, well worth calling in at Gigrin Farm for photo opportunuities of Red Kites, Buzzards, Ravens etc (plus other common birds) info here http://www.gigrin.co.uk .
Nick