I thought i had posted this for all of you but it appears to have disappeared!
so the museums association does a ticket for about 52.00 per year or less if you sign up for direct debit.
I just paid 12.50 to see the pre-rapelite exhibition so if you think you will be a regular visitor to the special exhibitions it is probably worth buying this ticket.
ALSO
if anyone is interested in visiting London galleries on a regular basis either in the week or at the weekends please let me know so that we can get together as i do believe that going solo to any exhibition is not a great idea where as going with someone, however different your opinions and backgrounds, in fact greater diversity is always a good thing, the more meaningful and greater the  appreciation and understanding can be.

http://www.museumsassociation.org/members-guide/london