The only changes Avanti have made, as far as I am aware, is scrapping the restrictions for travel to/from London Euston (EUS) on Fridays (the Friday evening restrictions were already abolished by Virgin Trains in their last days).
From Monday to Thursday the restrictions are the same as they've been for at least the last 10 or 15 years.
Restrictions from Lancaster (LAN) and north thereof are indeed significantly more lenient, but the restriction code for those journeys
(3A) forbids you from breaking your journey on the outward portion:
Train Managers are well aware of this 'loophole' and hence they are, for once, quite likely to charge you the excess if you've begun your journey at Preston (PRE).
That excess would be the difference to the cheapest fare that permits break of journey, i.e. the Anytime fare -
ouch.
Now in practice this is difficult to detect, because you're perfectly entitled to get a Northern or TPE service from LAN to PRE, and then to change onto your Avanti service.
But just occasionally, Virgin used to hand out slips to passengers boarding at LAN, to "prove" they started their journey there, and TMs would ask for this slip if you produced a LAN ticket. You'd have a bit of explaining to do if you had a LAN ticket but couldn't produce a slip! I imagine Avanti probably still do this occasionally.
There are still legitimate ways to make use of this 'loophole'. You could, of course, buy a return from PRE to LAN and actually travel up to LAN to legitimately begin your use of the LAN ticket (and collect a slip if they're being handed out) and vice versa on your way back. The inconvenience is sufficient to put most people off!
Or, seeing as the restrictions only apply on the outward portion, you could buy two Off-Peak Returns, the first from EUS-LAN and the second from LAN-EUS.
You'd then 'throw away' the outward portion of each ticket and only use the return - so using the EUS-LAN return portion on your way to London, and the LAN-EUS return portion on your way back north (assuming you want to travel during the time that PRE tickets have evening restrictions).
It feels a bit wasteful but at £210 it's still a lot cheaper than buying an Anytime Return at £378.60! Note that you're probably better off buying the first ticket (EUS-LAN return) to begin on the day before your journey to London - otherwise it's pretty obvious what you're doing! Whilst it's perfectly permitted, it's just liable to cause less hassle if you do it that way.