Whilst I agree that your little granddaughter will probably be fine, I have to declare that I was a Speech and Language Therapist specialising in early language development, and we would definitely be interested in a child who was not using several two word combinations by the second birthday, e.g. Daddy bye; my teddy; big doggy etc. In all probability, she is just a bit slow in developing spoken language, and as previous people have commented, will grow up to be entirely competent in language. If she can understand words spoken without visual cues (e.g. "show me your FOOT; where's the DOOR" etc., then her receptive language is coming on, and it is probably a delay in her expressive language only. However, it is worth getting her on a waiting list for assessment. There is no waiting list so long as the one you're not on yet, and if you wait several months and you're still worried, you'll have to start from scratch. Don't ask the Health Visitor, tell her (or him) that you want a referral, or ring your local speech and language therapy department. Sorry, but waiting lists are long, although some areas have a rapid initial assessment policy.
About 7% of children have language difficulties. People are naturally keen to tell you reassuring stories about language development, but just imagine if she had difficulty in motor development, would you hold back and wait to see if everything was OK? IF there's a problem, it's best dealt with early. If there's not a problem, what have you lost?
Meanwhile, you can look on-line at ICAN children's communication charity which has ideas for parents to encourage language development. I hope it goes well.