I'm fortunate in that my employer offers the option of either a corporate card or, if the employee prefers, reimbursement of expenses with any FX fees reimbursed with no questions. I consequently do the following:

GBP Sterling spend on BAPP AMEX wherever possible (1.5 Avios / £ or 3 Avios / £ with BA)
GBP Sterling non AMEX merchants, HSBC Premier World Elite Mastercard (1 Avios / £)
Other currencies (BA) BAPP AMEX (3 Avios / £)
Other currencies (non BA) HSBC Premier World Elite Mastercard (2 Avios / £)

So fees apart, I'm getting a return of a minimum of 1 Avios / £ through to 3 Avios / £ at the top end. For personal use I use Halifax Clarity whenever the currency is not GBP Sterling.