Being married to an ex RAF pilot and having an ex Army helicopter pilot son, I am a lover of planes and appreciate the service our pilots give. Our boys are very likely about to be going to peacekeep in Ukraine, or even fight bigger countries, so please try to accept that they have to practice their skills somewhere. Mostly they come up to Scotland to play in the hills.
I grew up in Norfolk and the jets made my Dad furious as every time one used our farm as their turning point, every turkey ran into the same corner of each house and so many were killed in the crush. He was in regular contact with the Station masters of the relevant local bases and they did try to limit over flights. Each turkey could only be killed once though.
I think if you approach the bases in the right way and tell them it's lambing time so your ewes are at a very vulnerable stage, they should respond by limiting pilots activities a bit. There is no way that a pilot flying at low level and high speed can notice sheep being stressed, even if they have the country skills to know it's lambing time, so please be understanding. There is a limit to how low they can fly and how close to buildings they can fly. If you can be precise in your complaint that will help. The exact time the plane flew low and the plane's identity number will help too. Then they know precisely who was the offender, rather than just giving a general telling off.
Our military services will be protecting us from very great risks all too soon. Hopefully your sheep will soon get used to overflying aircraft, but I suggest you bring them in for lambing.
It's an age old dilemma and I do hope you get some concessions.
Here we get a lot of low flying planes, helicopters and hot air balloons but luckily our Hebbies are pretty laid back about them.