The easiest way to work out what you can plant is to work out your growing season.

So for instance, the last frost is, the 1st November and the first frost is the 28 Feb (keeping this simple for my preggie brain).. so that gives us a growing seaons of 4 months or roughly 120 days (is my maths right?).

Seed companies such as eden, and I think on the back of the packets of commercial seeds, will tell you that the seeds will pop up in 7-21 days, and that you can harvest in, lets say 60 days. So all you have to do is work out how many days you have left in the growing season to figure out what you can grow.
The growing season depends on the soil temperature and when you have the first frost - so if you are willing to baby your vegies or create microclimates you can keep some vegies going a little longer.

Of course you can rely on the planting calendars, which can be useful, but i've been caught out a few times with their generalisations.

Depending how keen you want to get, you may want to keep a weather diary, where you record your first and last frosts.

Ohh and the question about the lettuce going to see - let it go to seed and try seed saving (depending on the variety - you might have seed for next season - if it isn't hybrid).