Visa valid from date you tell them =(

I hop eit does not cost too much to change the flight, i know cost is a big issue with us, so i always buy the cheapest ticket which is usually non refundable, but when i have had to change the date int he past they pn;y charged me £25, luckily the price of the ticket was not any dearer, fingers crossed , it will be as financially painless as possible for you.

My general feeling is that flights from Europe into the UK are a light tighter these days then they were even a few years ago.  I have gone in and out of several smaller airports and the laxness that was once there does seem to be gone.

On the other hand, if you do end up in the situation where you feel you need the stamp in your passport and you don't go through Borders, you can always stop at the point prior to going through customs and ask someone.  There have been a few instances on the board where people have done such that and roused the 1 Borders person from their sleep to stamp their passport. [EDIT]  I just saw some cases where this doesn't work...  So take my advice with a grain of salt.

When you are flying within the Common Travel Area (CTA) of the Republic, IOM and the Channel Islands and the UK, you often get spotty service though again, you can usually demand it if you know you need it.

The big thing is being forewarned that you need the stamp to validate your Visa.  Don't be afraid to be a pain to airline and airport staff if you are unsure.

Ok, now I'm worried, DH, DD, MIL & I are heading back to Cali for Thanksgiving, United has now moved all operations to Terminal 1...am I missing something here? I think I should be OK because my original entry stamp was Terminal 3 back in February.

If you are flying directly from the US into the UK, then you should be fine. I flew into Terminal 1 with United (from Washington DC) last month and I went through immigration in Terminal 1 just like you would have done when you landed at Terminal 3 in February.