It is only beneficial if you have nominet membership also.
So the 80 domains you have, say you pay your registrars £7 each for renewals, that is £1 more than you would spend with nominet direct.
So £40 a year more.
It costs £120 a year to stay a nominet member, so not worth it on that basis.
If however, you intend to buy more domains and even though I doubt it will happen myself, the dot uk is launched, you will probably save more.
So taking a standard 4 year roi for fees etc, I guess 450-500 domains would make it beneficial and cost effective.
Ps
If you do see yourself getting to those levels over the next year or two then its makes sense to get membership tag as soon as its economically possible.