And your problem with that is ....?

The real problem is the cost. Annual fee is £150 plus the interest rate on £100k of 4.7% is 2% less than you can get elsewhere. Assuming you're a 40% taxpayer and do this purely for the miles, the cost of your 50,000 miles (assuming you stay with them for 13 months) is £150 + £150 + £1,200 net lost interest = £1,500.

Even comparing with the best Sterling instant access account (the best of which pay 6.2%) you are still 'spending' £150 + £150 + £900 = £1,200 for 50,000 miles, which at 2.4p per mile obviously makes no sense.

When I did this back in 2005ish the spread was a lot lower so the maths did make sense.

Perhaps a better deal is to drop in £100k, pay the £150 fee and then close the account as soon as the miles have hit. This reduces the lost interest to a more manageable amount. It is still not a no-brainer, though.