Can I ask an entry level question...

Please don't slap me for it...

On Tesco CC web site, it says that £2.50 of CC vouchers gets you 600 BA miles, but when you click on the amount of vouchers you want to change to BA miles, it goes up in increments of 50p...

Can you only exchange them in amounts of £2.50?
Or other denominations?

Thanks,