In most situations, the two are very different...
Doctors in General Practice are usually self-employed or in small partnerships (no political discussion of recent NHS changes, please) and when one is absent, the practice is unlikely to be able to fill the void themselves.
On the other hand, ICT companies usually have enough staff to be able to cover (temporary) absence. This also tends to apply in most hospitals.
There are two specific situations when "locum" staff are used within ICT:
- For long-term absence (whether due to sickness or perhaps maternity leave) then it is not uncommon for a company to bring in an agency temp as cover.
- Extremely rarely, a freelancer has been allowed to supply a substitute worker. Substitution was seen as a silver bullet against the UK's ir35 laws, but IME client companies are seldom amenable.
Whilst in the NHS, some doctors make a good living out of being a career locum, the ICT career locum is not a business model I would have confidence in being lucrative for an ICT worker.