Yes, voting by post is safe and proven cases of electoral fraud are rare. When voting by post, you should mark your vote on the ballot paper in secret, and seal the envelope yourself.
You will also be asked to give your date of birth and signature when applying for a postal or proxy vote. This makes postal voting safe, because when you return your postal voting pack your signature and date of birth are checked against those you provided before to confirm your identity.
Your signature and date of birth are separated from your ballot paper before it is looked at or counted, so giving this information will not affect the secrecy of your vote.