If you are looking for Large picture frames in the UK and online you might want to consider asking your local picture framer for a quote before ordering.
Choosing a frame is like choosing clothes. You are making an aesthetic judgment about style and compatibility and its right to want to get it right!
Buying any large item will cost more than its smaller counterpart. In the case of frames the downside of ordering online is not being able to see the molding beforehand.
A large frame ordered online will also have more chances to be damaged in transit than a smaller item and obviously the larger the frame the more costly the postage.
Conclusion! With the increased cost of buying a large frame and the uncertainty of the molding and possible damage in transit I would always approach your local picture framer first to get a quote. You might be pleasantly surprised!