I believe if declassified, the compensation is the difference between 1st and Standard equivalent fares.
I went 1st class to Newcastle the other day, as I got the tickets under the 25% off offer. At peak times, I would say 1st will give you a better chance of a seat (and more space too) than standard, but at quieter times, unless the fares are similar, it probably isn't worth it.
Off the Scottish routes, catering is only provided on the core between Manchester and York and only until 7pm. Thus, on my recent journey to Newcastle, I managed to get a drink and cake before the trolley bailed at York, on the way back (18:52 off Newcastle) there was no catering, but there were only 2/3 of us in the entire 1st class section, so it was mostly very quiet and relaxing.
You say 1st is double of standard...given they are AP's, it depends on what standard price is, as fares may be currently very reasonable...depends what you double as to whether it is expensive or not :P