Welcome to DakkaDakka newcomer!
Ultramarines are a great idea as a start point as you said, especially if you're considering to get Betrayal at Calth; I don't know much about 30K, but
BaC is certainly much cheaper than buying
FW resin to start playing, plus those models can now be used in
40K under the Angels of Death codex for Space Marines. Good stuff all round!
The Start Collecting sets are also great value, and buying two will allow you and your son to get playing straight away, as they all include formations for immediate use in the game. So really it's up to you - either split Betrayal at Calth or get a couple of those
I know a good site based in Cheshire that sell
40K at a 15-20% discount across the board, and who deliver really quick across the
UK - check them out
here, I highly recommend them
Unfortunately I'm in Bournemouth and am trying to settle down after just returning to the
UK after a decade of living abroad, and I don't play anymore either. However, be sure to check out the
player finder to get some results for people you can play with.
Hope this helps
Happy hunting!
G.A