Ah but ....
The Amex 2.75% loading equates to a cost of £2.75 for 150 BA Miles on a £100 spend, which is 1.8p per mile. Not great but in-line with what BA charges.
On a Travelex triple miles promo, £100 of spend gets 300 BA Miles for a spread of probably 3-4% over the rate Nationwide uses. In this case, you are better off using TC's since you'd be 'paying' 1p per mile, which is very good.
However .... there is no-way that I could hit my £15k Amex spend without putting all my holiday spends onto it. This is the main reason I don't get a Nationwide (or indeed Saga) Visa.