FTB Need help understanding re-mortgaging and how to choose a mortgage term

Hi there, my husband and I are immigrants to the UK and are jumping in head first into the home buying process. All that to say, not only is the home buying process totally new to us, most things about the UK financially are new to us. We also both come from working class backgrounds and did not grow up with great financial literacy although we're learning a lot as adults.

Okay, so, we're looking for mortgages currently. We're looking at houses up to £450k, we have a 90k deposit. FTB. We are seeing rates around 4.5 for 2 or 5 year fixed, 5.5 for 10 year fixed. Questions:

1. What is the upside of remortgaging after a 5 year fixed as opposed to taking a longer term mortgage? When we originally started looking into the home buying process, we saw some decent deals for "life of mortgage" rates. To me, depending on the rate, locking in a rate over the life of the mortgage is not a bad idea. But I've read that remortgaging is basically always better in the long term if you build up significant equity in the home over the course of your initial mortgage. How does this work? I just can't understand how it could be better if, for example, the rates get worse. Are we just assuming the rate will be better in 5 years, or is there some fundamental reason why it's always better to remortgage rather than have a longer period mortgage?

2. Overpayment / length of loan term. We could have a shorter loan period of 25 years (mortgage advisor is offering us a 30 year period). However, does the loan term really matter if we are planning to overpay? I think there is a penalty associated with overpaying too much, so is there some kind of math to be worked at re: how much you can overpay versus the rates you can get at various terms? Does the length of the loan even impact the rate?

Thank you for your time and advice, sorry that I am not better read on these things.