Rain is fine. But, does the line DC during or after it has rained? If so, there's very likely an issue on the line, and a call to get a BT engineer out will be required to resolve it.
If it is DC'ing even without rain, then roughly what times during the day? Or is it happening all day long?
Also, what line stats are you getting for your FTTC connection? Signal Attenuation, Line Attenuation, sync speeds, etc.
If you had a phone you could plug into the line, you could use the quiet line test and see if the lins is super crackly or with lots of static. If so, that could heavily impact on line stability and connection itself, causing it to DC. Again, this will need an Engineer call out, but at least you know it'll be an actual fault and thus won't be charged for the call out.