Fly BA to Glasgow then BA to Stornoway, Isle of Lewis.
Option to go via Edinburgh.
You can through ticket with the one booking this way with BA.
Individual flight to Lewis is about �100 in summer from Glasgow, so you may find an Easyjet and BA combination cheaper at some times - depends on your travel dates. BMI also fly to Lewis from Edinburgh so you have a couple of options with carriers and rates to check.
Highland Airways flies Inverness to Lewis, but BA to Inverness is more expensive than to Edinburgh, so check your routes carefully.
Or fly to Glasgow / Edinburgh and take the train to Fort William, bus on the same combined ticket to Ullapool and take the CalMac ferry it to Lewis......much more pretty way to do it....