Parcelforce is Royal Mail's courier service basically. When parcels get to a certain size, weight, value you are forced to use Parcelforce instead [or the Royal Mail simply will not carry it]. You can have something that size shipped with Parcelforce easily. They are normally not the cheapest service but neither are they the most expensive. Typically Parcelforce sit in the middle of shipping price averages.
Despite being more expensive, I will admit I do prefer to use them as it means you are shipping with the post office and shipping with one party. A lot of the cheap courier comparison sites involve middle men and sometimes you dont know who is actually dealing with your parcel; if you use it make sure you pick a recognisable courier. I have had as many bad experiences using these sites as good ones...unless I take the time to pick a courier I know will deal with the parcel themselves without a sub contractor involved.
I do recommend using one of the courier comparison sites and shipping as one parcel; as you are using a courier, shipping weight is not as important any more [as long as it does not get too high]. You should manage to get it to the US for the £30-35 mark.