I live in the UK and am a member of both BA and BMI's programme (as well as others). Here is my short breakdown for you:

BA's programme still has many aspects leftover from when it was only open to business travellers (ie full fare Y or premium cabin travellers). This means that if you travel a lot in low fare Y then the programme is far less generous than other programmes. However you still have good opportunities to earn BA miles through Tesco points and BA Amex spend (won't be possible to get till you start your job).

BMI's programme is still probably one of the best out there (but may not be around for long due to LH takeover). The programme like most european programmes doesn't offer 100% mileage on low fare Y, but the programme is still very advantageous for low fare Y travellers due to the great redemption rates. The BMI Amex is probably the best credit card in the UK for miles. The standard card has no annual fee and the BMI Plus Amex has a 60 GBP annual fee (low compared to other cards in the UK). I have the BMI Plus as I find the extra bonus miles and higher earn on cc spend to be worth the annual fee, but you will need to calculate whether your spend will justify it.

As already mentioned you have the option to sign up to the US programmes or even Air Canada's programme, but there are some advantages to being signed up with the UK based programmes when based here in the UK. Unfortunately it isn't a calculation that is straight forward and often depends on what you value most and what your travel profile looks like.

Let me highlight some of the general differences:

- US programmes are more generous on the earning (some methods only available for US residents)
- US programmes give elites either automatic or "sticker" upgrades for domestic flights (some also for international)
- US programmes don't charge as high tax / fuel surcharges on awards (BA vs AA)
- UK programmes have better award availability (BA vs AA)
- UK programmes have better award charts (BMI)
- UK programmes have more earning channels for UK residents

Now if you choose to go Star Alliance I would definitely go with BMI Diamond Club, but if you go with OneWorld it would be a more difficult choice for me. Depending perhaps what level of travel you expect to have with your new job. AA will definitely be better if you expect to be doing mostly low fare Y (ie mostly personal travel) as you could do a Plat challenge to get some status out of your RTW, but remember that it will mean no status on BA transatlantic flights)!