The best product for your lip would be determined at your consultation with a board certified plastic surgeon. We would assess your lips and do a thorough consultation and understand the look you are going for then pick our product to achieve the most natural, beautiful result.
Which filler for the lips
I have used various fillers in the lips, and my favourite has been Juvederm Ultra, although Juvederm Ultra Plus or Restylane can also be good options. Teosyal has been used successfully in Canada, and will soon be available in the U.S. More importantly is finding the right injector who understands lip anatomy, symmetry, beauty, etc., so look for a highly trained skin expert such as a dermatologist.
Teosyal is a hyaluronic acid filler not available in the US, but should be an excellent choice if the injector you choose uses it and has excellent results with it. The challenge is selecting an injector. That physician should be an expert who has done filler injections for a significant time and understands the anatomy where you wish it placed. (See ExpertInjector.org)
Teosyal has not yet been approved by the FDA in the United States. You may want to check on its approval in the UK. Juvederm or any other approved hyaluronic acid filler will do the job. The more important factor is your doctor and her/his technique.
IMO..juvederm is the best product for the lips for numerous reasons. However, the main ingredient and the most crucial is finding the right injector. This person needs to understand the anatomy well. Hope this helps. Good luck.