Hi Pauline,
You should be able to check your luggage straight through to Boston when you check in at Manchester, and you should receive two boarding passes one for Manchester to Heathrow and Heathrow to Boston.
My colleague has asked me to advise you to confirm with the BA representative at the desk that they have checked your luggage straight through.
In terms of currency, this is at your discretion - you will be spending slightly more time in the USA and you are mainly budgeting for food and souvenirs as many of the excursions are included in the price of your holiday. It is worth mentioning that credit/debit cards are widely accepted in North America, but please notify your card issuer before you leave the UK.
Kindest regards,
Julia