what can we expect in way of lines while in London in mid march???

we will going to Tower of london, London Eye, museums.

In our experience, we didn't find lines bad at all in March. Regardless, I'd always plan to go to the Tower of London when it opens since it consistently seems to be very popular.

Lines really don't apply to most tourist attractions in London. Exceptions would be the Tower of London (yes, arrive there just before opening time to get right in), Madame Tussaud's (best avoided by most visitors - but they must come from somewhere since the queue can be around the block), Westminster Abbey at opening time, and a very few other places. Since most museums/galleries have free entry, there are no ticket windows and usually several entrances -- so queues are not an issue.

The front courtyard at the British Museum can be crowded at/just after opening time because folks arrive before the doors open. But after that - you really just walk in.

At St Paul's there is seldom a line of any length to worry about - but there can be a delay for the security check

Not sure if you plan any day trips out of town, but you might have to watch out for early closings if you do. On a Dec. '04 trip we were not aware that Dover Castle closes early and didn't particularly rush to get there. Would like to have had another hour or two because we really liked it, though I didn't particularly expect to. In Dec. it was dark around 4PM, though in March it should be light longer. Seem to recall winter hours were a bit shorter than I expected at a couple of other places.

from what I have researched there should be about 11.5 hours of day light -6:30 - 6:00

but with the floodlights will even sunset be much of an issue???

is there anything that is a must during daylight??

what about lines at heathrow and for tube and buses? yes i know London is full of residents but as far as tourist in march - is it less crowded then?

March will likely be a less crowded month but there are always many tourists in London. The time of March that might be more crowded is the last week, which is spring break week for many U.S. schools so lots of American families visit and if Easter is in March (which it was last year but is not this year) which means it is a break also for many UK and other European schools.

We didn't experience any lines at historical sites, churches or museums the first week of April last year.