What I'd do is take a flight from BRS with bmi as long as you're sure that flight won't affect your requalification.

That'll give you an idea how well the public transport links to BRS now work. There's also a few bmi regional trip reports in the bmi forum.

You also have to think of the Tesco slogan 'every little helps', if you can see in the future a time where you're likely to be earning more tier points then the ones you've earned already would help you reach lifetime gold sooner, or if they decide to launch a lifetime silver in future that might turn out to be in reach.

In your personal situation I would decide (after trying the BM flight) whether the convenience (assuming there is any) outweighs the benefits of BA (better lounges, Avios, etc).

Just to throw another option into the mix, if you do decide to stick with BM you could always get a years Cathy Pacific gold membership (equivalent to BA silver), with an American Express Platinum Card. The annual fee is £450, but if you can't justify that cost you can actually cancel the Amex and claim a pro-rated refund on your Amex fee whilst the CX Gold remains valid until expiry - it's a good idea to also qualify for any signup bonuses before you cancel too, as they'd get you more membership rewards points that can be converted to Avios.

The CX Gold is officially for a year, but most Amex Platinum members (who keep the card) have managed to renew theirs every year so far. The benefit is not widely promoted on the Amex site due to Amex also having a partnership with BA.