Hi All.
To clarify a few things re the airport for you...(I work in airport security)
The archway is a magnet - not an xray. It will not affect your pump, neither will your pump set the alarms off. Your mobile phone will though - so take it out of your pocket!
There are two different alarms - one is random (even naked you would set it off) the other has detected metal dense enough - usually a belt, mobile phone, high heal shoes, large buckles etc... Not your bra ladies!
You may be asked if it is ok to swab the pump if you are selected to be searched - this will not damage the pump. Neither will the hand held metal detector - again, this is just a magnet.
The xray itself emits less radiation than is experienced on the aircraft. This will not damage or have any effect on insulin, BG monitors or pumps (if you took it off - but why would you anyway!)
With regards to needles and insulin in your bag - Security are only interested in this if your bag is selected for search. You do not need to tell them unless you have a cool bag with an ice pack in your bag - in which case, you should take it out of your bag before the bag goes through the xray machine. You do not need to use the clear bags for your insulin
The body scanners are infra-red, so will not affect your medical equipment in any way.
At Gatwick, these are used randomly after you have cleared Security. You can refuse to use it if you are asked. But be prepared to speak to a supervisor as to why you don't want to use it.
Letters are sometimes required by the airline - I've yet to see an airline ask for one though!
It can be handy to have your prescription with you though - especially if you have prescribed creams/gels etc that are more than 100ml. These will be allowed through security if you have the label on and it corresponds to the prescription.