If my brother acts as guarantor for my mortgage, but does not put his name on the house deeds, would he no longer be classed as a first-time-buyer? This is important because it makes a difference to whether or not he pays stamp duty when he buys his own house.
To be classified as a first-time buyer you must never have owned a residential property in the UK or abroad. This includes freeholds and interests in leaseholds.
Being a guarantor on a mortgage is not "owning a property", having your name on the deeds at the Land Registry is.
However, your brother should get his own solicitor (the mortgage company may insist on this), who can confirm my understanding is correct. More importantly, the solicitor can explain the risks of being a guarantor to him - he may decide to back out. (Having said which, I acted as guarantor for my son when he rented as a student.)