We have flown with Jet2 ( http://www.jet2.com/ ) on three holidays, so six flights in total. We flew from Edinburgh to Faro in Portugal two years ago and more recently from Edinburgh to Lanzarote December 2016.
We informed the airline before our travel date that my son had a severe nut allergy. The first time we flew, the crew had no note of this but were very welcoming. They were happy to make an announcement that they would not be serving nuts on the flight and to ask passengers not to eat nuts they had brought on board themselves.
On our subsequent flights, the crew have been aware of his nut allergy and have not loaded nut products onto the plane. All staff/crew have been friendly and efficient when dealing with our request for nuts not to be served.
We have not requested early boarding (you can pay extra to do this if you prefer). I cleaned the seat and surrounding area upon boarding. My son is small enough to stand out the way while I do this. We paid extra to sit together. Early morning flights are best as the plane is at its cleanest!