If one of my customers said they were losing signal on BBC1 & 2 then I wouldn't be thinking they need a booster or a new aerial at all.
The digital channels are broadcast in
groups that are known as multiplexes - Mux for short. Stations that have to achieve almost total UK coverage are broadcast with the highest power. These are the muxes that include BBC1 & 2. If you are getting break up on the strongest channels, but still receiving the weaker ones okay then the aerial is probably okay.
A good first step with any of these kind of vague and spurious problems is to do a "first time install" tuning. This will clear the existing channel memories and retune from fresh.
Next, have a look at the signal strength. Not every TV and Freeview box measures to exactly the same level, so use a bit of latitude here. But essentially you need only about 75% signal strength and then as much signal
quality as possible. Quality is the most important one. It's a measure of signal against background noise. The better the signal quality then the more signal you have versus background noise. This is why boosters aren't a universal fix. If you have a lousy low powered signal then a booster will make it a lousy high powered signal.
If you find your signal strength at max then you might have too much signal and that can be just as bad as too little. An attenuator can reduce the signal level if required.
If these aren't the issues or you are still having problems then check your cable. Coax doesn't last forever; particularly outdoors. If the outer sheath is worn through or cracked then water can get in to the cable. Also look for areas where the cable goes through a tight corner. Sharp bends can create a notch filter that affects some signals but leave others untouched.