The High Commission of India has outsourced the OCI application submission services in the UK to VFS Global (likeIndian passport surrenderandIndian passport renewal). Consequently, applying for OCI by post is not an option and you have to do it via only VFS in the UK (and CKGS in the USA...
Additionally, if you have ever held Indian citizenship then you will need to submit proof of renunciation of that status when submitting your application. It is also important to note that some countries require additional forms and documents specific to their country so it is best to check with...