There are no formal restrictions on adding, dropping, or changing a middle name. Simply apply for a Deed Poll with us and add a middle name of your choice. Once you receive your Deed Poll, all you need to do is change your official documents to show your middle name as a part of your legal name.
Please be advised, unless all your documents are changed, your new name will not be recognized as your legal name. Mere possession of your Deed Poll does not give your name a legal recognition. Although there is no immediate requirement for you to change all your legal documents (like drivers license, passport etc.), if you are planning on migrating to another country, it is imperative that all your documents share the same legal name.
You may add more than one middle name to your name, however, you should be aware that HM Passport Office can only print a maximum of 30 characters as your Christian name (and this includes your middle names), and a maximum of 30 characters as your surname. If your new Christian name and/or middle names exceed the limit, you should request that they add an abbreviated form in the main part of your new passport, and that they print the names in their entirety on the observations page.