Depends on what you are doing. Austrian alpine gives the basics and if you have the equivalent of an NHS card ( free health care throughout the EU) then you should go with that. Its a single payment per year of around 30 quid.
The alternative is to join MI as its cheaper than the BMC and you have access to the insurance. This insurance is not covered by membership and is on a trip by trip basis. If you are going to Spain for a week then you will need to purchase insurance for that week which covers roped climbing ( standard travel insurance doesn’t cover this). This will be around 30 quid a trip.
So to conclude.
If you are going on a one off trip, join MI, purchase single trip insurance.
If King Willy of O provides you with free European health insurance cover then join Austrian alpine club
If King Willy of O doesn’t then join MI and purchase trip insurance per trip