It really depends on your school which you picked out to apply to. I myself just finished my A levels and have moved to England for the beginning of Y12. It is important that the school you pick is flexible regarding the shift in educational system transfer, many schools will ask you to sit GCSEs to prove that you are on the same level as the others to provide a 'fair sit'. However, as much as I understand the logic behind this enquiry there is a very big lack of trust towards your original country's educational system. Additionally asking a foreigner to sit an exam which is purely based on UK students' curriculum and syllabus is pretty unfair. You need to be aware of the fact that the transcript your previous school has provided should be efficient and a valid proof of your potential and ability no matter what language you studied it before. Yes, the school may require you to show proof of english language proficiency but if the school is immediately denying the acceptance of your transcripts I would go looking for a new sixth form. I must emphasise though that the sixth forms are not obligated to accept the foreigners transcripts so it might cause some problems. However, this should be a heads up for you that even if you decide to sit the GCSEs, in the future the school will prove to be just as inflexible and unhelpful when applying to uni as an international student.
Coming from a different country and a different educational system will result in you starting 6th form from an unadvantaged position. It will be hard and difficult as you previously have studied a different syllabus and core colloquia. You will not have the same base of understanding as the others and on top of having to study in a new language (that said if you are incoming from a non-english speaking country) it will feel like learning a new language with the specific vocabulary you'll have to get your head around. HOWEVER, it is well worth the energy and time that you'll need to put into getting the good A level grades. I myself and other foreigners who have been in a similar position to you got into sixth form without GCSEs. The initial trust from the sixth form head when starting such a new 'life' and leap of faith is essential. Please make sure you are comfortable with the school you choose as they will be the ones guiding you through these 2 years, the last 2 before uni.
I would love to help and share my experiences on this topic as you can see just on this one aspect of choosing the right sixth form resulted in a huge paragraph... Please feel free to reach out and contact me if you feel I would be of any help or anyone else here who answers as I am sure we are all happy to help as we've been there and done it and every little bit of information helps and is appreciated.