I'm sorry you are having these issues.
Assessing your "before" pictures would be helpful in determining the cause of the fullness after your lower facelift. However, it is more than likely you have a seroma in the area. I would strongly recommend seeing a plastic surgeon, general surgeon, or if possible the surgeon who performed the procedure, and an aspiration (with a needle) would more than likely help remove the fluid. Delaying treatment could cause the fluid collection to harden and calcify, leading to aesthetic issues or it could become infected.
I hope this helps.
I would try to aspirate this area of swelling if I felt some fluid underneath the skin. Consult a board certified plastic or general surgeon near you. The procedure is a simple office procedure involving a needle stick and a syringe under sterile circumstances followed by a small elastic dressing. Be sure to consult a surgeon with a proper amount of experience and qualifications
Without an exam it is impossible to make a diagnosis or give advice other than see a plastic surgeon (preferably your own surgeon) with expericence in facelifting.
A seroma is a straw colored fluid that accumulates in a surgical space like a neck lift. The fluid may absorb slowly but aspiration or drainage can allow it to heal much faster. In my experience many seromas need to be drained multiple times and the more frequently it is drained, the faster it will resolve.
Seek out a plastic surgeon in your neck of the woods and it will certainly be taken care of properly.
A plastic surgeon should not need to order an ultrasound to see if there is blood or fluid in a persistent area of swelling after a facelift. Instead, needle aspiration is both diagnostic and therapeutic. The skin is cleansed with an antiseptic and, after a small injection of local anesthetic, a needle attached to a syringe is introduced into the area of concern. The stopper on the syringe is pulled back and any blood or fluid in the area will be drawn into the syringe. After aspiration, a pressure dressing is typically applied for a day or two.
Although it is always preferred to have your original surgeon continue to treat you in the post-operative period, a general surgeon in your area should have the know-how to aspirate under your chin.
If you can not get back to your Plastic Surgeon, see one in your area. You are a good example of why you should stay close to your surgeon for post op follow ups. Best wishes!
If your ultrasound that ruled out a hematoma or fluid collection (seroma) then you may simply have inadequate correction of the loose skin or residual fat beneath the chin.
I strongly suggest you consult a board certified plastic surgeon and the UK.