Last I checked Royal Mail didn't have an API. To use them you have to get their price charts, populate a database, and then look up the associated charge from the recordset. I haven't done ParcelForce, but assuming they have sample code you can use that works, it wouldn't be too difficult to implement (we could help with either of these in a premier support ticket)
Quantity based shipping is a simple rule you can add to the cart. I'm sure you could do that one on your own by going to the shipping tab of the cart object and adding a rule for each price point.
Did this help? Tips are appreciated...
