The bad news is that, unless you've been quoted Advance fares (for specific dates which you've not mentioned in your question), the fare of 'around £87' referred to in your question is the SINGLE ('cross-country) fare from Norwich to Blackpool, not the Return one. (That's the 'Anytime' fare. There doesn't appear to be an Off-Peak Single fare available).
The Return fare on the cross-country route is £143 using an Anytime ticket, or £112 travelling Off-Peak.
Via London an Off-Peak Single ticket is £121.50, with an Anytime Single costing £202.90. There are no Return fares, per se, available, so you'd have to pay in each direction.
It's possible (but not necessarily very likely) that you might be able to get cheaper rail travel by combining Advance tickets but that would probably only apply (if at all) if your travel dates were well in the future. For example, on certain trains you can get to London Liverpool Street for £15 single but I'm unable to find any equally low fares from Euston to Blackpool. I doubt that you'd find any ticket combination cheaper than the normal £112 Off-Peak fare. (If you provide your travel dates I take a look for you though).
Megabus can get you from Norwich to London. (There are no cross-country routes available). Leaving at 0630, you get to London at 1000. (3½ hours). The fare, when booked a reasonable way in advance, is £1.
From London, Megabus will take you to Preston. with the 1200 departure getting in at 1825 (approx 6½ hours). On a random date I've just chosen in July, the fare offered is £12.
Coming back from Preston, you could get the 1150 service, getting into London at 1730, then the 1930 service arriving in Norwich at 2245. (£8 + £1 on my sample dates). That would make £22 return from Norwich to Preston.
20:37 Tue 21st Jun 2011