No, you do not need a solicitor. Your witness can be anyone who is at least 18,
knows you, and is independent (not a relative, partner, or someone living with
you). A friend, neighbour, or work colleague works perfectly.
If you’re not British and live in the UK, check with your Embassy to see if your
deed poll requires a solicitor or notary public. And if an Apostille is needed,
it must be witnessed by a practicing UK solicitor or notary public.