If you can get across the top I can promise some of the most fantastic flying you can see. It will be a most rewarding trip. I fly alot in that area and there are a couple of points to consider.
You are kind of in 'No Mans land' in terms of radio. Try London FIS but over the mountains you may well lose them if you are not high enough. Filing a flight plan may be worthwhile if even just for peace of mind, as previously mentioned don't forget to close it manually on arrival or expect a rather steep bill!! Note too that Mona doesn't have any AVGAS available and Caernarfon is your closest option for that (and reasonably priced if I remember) so make sure you have enough to get back there or top up on the way! Valley Radar doesn't operate at weekends and will not provide you with any service. The AIAA is dead quiet over the weekend. I would suggest you call Caernarfon Radio for traffic info as they get really busy and will let you know who's where around the Straits. There is a Flying Club at Mona who I am sure will assist you. Have a great trip!