I'd go the northern route personally. Lyneham are excellent, never yet had a Averbury-Chippenham CTR transit refused.
You should then route to the Severn Bridge and follow the Welsh coast past Newport to Cardiff Bay. Ask Cardiff approach for a standard VFR join via Cardiff Docks. Route towards Barry (not above 1500')and you'll be handed off to Tower early & given a right base join for 30/ left downwind join for 12.
If you do go the southern route, and 30 is in use at Cardiff, you will probably be requested to cross the Severn at Weston Supermare, remaining clear of the Bristol CTA, and to the East of Flat Holm. Altitude is again restricted to 1500', which is a bit low for my liking for a wideish river crossing, hence my preference for the northern route.
Ask to be 'handled' by the flying club on the south side...it'll save you a few quid.
And watch out for all the BA Jumbos parked on taxiway Echo - I think about 7 are there at the moment!
Enjoy the flight!
