Can an under-18 umpire take a junior match on their own?

No. Under England Hockey's Junior Competition Regulations (Under 14 to Under 18 outdoor, 2026/27, reg 2.3.7), when a Young Umpire (under 18) is appointed, the appointing club must make sure a qualified adult umpire is present for the whole match, on the pitch or on the sideline, to support them.

The same regulations set the umpire accreditation levels for junior league matches:

If an appointed umpire doesn't meet the acceptable level, the game can only go ahead if both teams agree. Umpires don't rule on competition regulations, and the competition organisers don't adjudicate on umpires' decisions.