Fixed rate mortgages and future job changes + remortgaging

[Vested Disinterest]

I will probably need to change jobs in about 18 months to two years.  I don't actually know what I'm worth on the open jobs market right now, but there's always chance I could be taking a haircut. I'm a cautious person, generally. Have two school age children...  I'm pretty handy with computers though, so I don't think I'll be unemployable.

I'm currently looking at choosing between a 2 or 5 year fix for a first time purchase.

My mortgage adviser says the mortgage is portable (as long as you don't downsize too much!) - so I'm leaning towards the five year, but the two year is maybe £50/m cheaper and the broker would take some of the bureaucratic pain out of remortgaging.

My question is: if I changed jobs just before the remortgage, could I be turned down for it?  Or do they take my whole employment history into consideration?  Do I need to be careful about the timing of elective job moves?

thanks!