TLDR; Writing a software licence is not considered "practicing law" so it is legal to do so without being a qualified lawyer.
The relevant governing law is the Legal Services Act 2007.
Section 12 sets out the "reserved legal activities" and sections 13, 14, 18, and 19 provide that it is an offence to carry on a reserved legal activity unless you are authorised (e.g. a solicitor) or exempt.
The reserved legal activities are (emphasis mine):
(a) the exercise of a right of audience;
(b) the conduct of litigation;
(c) reserved instrument activities;
(d) probate activities;
(e) notarial activities;
(f) the administration of oaths.
Schedule 2 defines what is meant by each of the reserved legal activities and paragraph 5 covers reserved instrument activities. The following provisions are relevant (emphasis mine):
5(1) “Reserved instrument activities” means —
(c) preparing any other instrument relating to real or personal
estate for the purposes of the law of England and Wales or
instrument relating to court proceedings in England and Wales.
5(3) In this paragraph “instrument” includes a contract for the sale or
other disposition of land (except a contract to grant a short lease),
but does not include -
(a) a will or other testamentary instrument,
(b) an agreement not intended to be executed as a deed, other than
a contract that is included by virtue of the preceding provisions of
Software licenses are almost invariably executed as simple contracts rather than deeds (you will know if it is a deed because there are specific formalities for that such as signature witnesses). Even for the obscure case where you were drafting a deed, there is the following exemption available in paragraph 3(10) of Schedule 3:
The person is exempt if the person is an individual who carries on the
activity otherwise than for, or in expectation of, any fee, gain or