You just got a bad agent.
That fare basis allows for a change with fee of 120 GBP.
Alternatively, you may cancel it, and apply the full value, minus 120 GBP, to a new ticket for travel within a year.
I looked up the rules on ExpertFlyer for WKXNCGBW.
Relevant section:
CANCELLATIONS
ANY TIME
TICKET IS NON-REFUNDABLE.
NOTE - TEXT BELOW NOT VALIDATED FOR AUTOPRICING.
--
A. UA CUSTOMER COMMITMENT
--
CUSTOMERS WHO PURCHASE A TICKET FOR TRAVEL
ON UNITED VIA UA CHANNELS MAY CANCEL THEIR
RESERVATIONS/REFUND TICKET WITHOUT PENALTY
WITHIN 24HRS OF PURCHASE.
--
B. EXCHANGE - CHANGE TO 1ST TICKETED FLIGHT /
WHOLLY UNUSED TICKET
--
FULL VALUE OF A WHOLLY UNUSED TICKET MAY BE
APPLIED TOWARDS PURCHASE OF A NEW TICKET PRICED
AT -CURRENT- EQUAL OR HIGHER FARE LEVELS UPON
PAYMENT OF CHANGE FEE CONTAINED WITHIN THIS
RULE AND ANY DIFFERENCE IN FARE.
--PROVIDED--
--ORIGINAL RESERVATIONS ARE CANCELED PRIOR TO
ORIGINALLY SCHEDULE FLIGHT --- AND ---
RESERVATIONS FOR NEW ITINERARY ARE MADE AND
TICKETS REISSUED NO LATER THAN 1 YEAR FROM
DATE OF ISSUE ON ORIGINAL PURCHASED TICKET.
--
THE NON-REFUNDABLE CANCELLATION AMOUNT OF THE
TICKET BEING EXCHANGED REMAINS NON-REFUNDABLE
AND CARRIED FORWARD IN THE ENDORSEMENT BOX OF
THE NEW TICKET.
--
C. LOWER FARE INTRODUCED OR AVAILABLE IN MARKET
--
WHEN A LOWER FARE IS INTRODUCED INTO THE MARKET
OR A CURRENT FARE IS REDUCED / OR BECOMES
AVAILABLE / THE TICKETED PASSENGER MAY
QUALIFY FOR THE ABOVE. ALL PROVISIONS OF THE
LOWER AIR FARE MUST BE MET BEFORE DEPARTURE.
NO CHANGE TO ORIGIN/DESTINATION/DATES/FLIGHTS.
ONLY BOOKING CODE CHANGE IS PERMITTED.
--
A SERVICE FEE EQUAL TO THE CHANGE FEE
WILL BE ASSESSED. -- RESIDUAL VALUE /IF ANY /
WILL BE RETURNED TO THE ORIGINAL FORM OF
PAYMENT.
--
D. EMERGENCY PROVISION
--
TICKET MAY BE USED TOWARD PURCHASE OF
ANOTHER TICKET DUE TO AN EMERGENCY OF
PASSENGER / IMMEDIATE FAMILY MEMBER OR
TRAVELING COMPANION.
DOCUMENTATION REQUIRED.
--------
THE OPTIONAL PAYMENT CHARGE WILL NOT BE REFUNDED.
CHANGES
ANY TIME
CHARGE GBP 120.00 FOR REISSUE/REVALIDATION.
NOTE - TEXT BELOW NOT VALIDATED FOR AUTOPRICING.
-- INFANT DISCOUNT APPLICABLE // CHILD DISCOUNT
APPLICABLE // CHILD DISCOUNT APPLICABLE TO INFANT
WITH A SEAT
--
A. CHANGE MADE TO OTHER THAN THE FIRST TICKETED
FLIGHT
--
TICKETS MUST BE REISSUED WHEN ANY VOLUNTARY
CHANGE IS MADE. WHEN TICKETED RESERVAIONS
ARE CANCELLED PRIOR TO FLIGHT TIME - NEW
FLIGHTS MUST BE CONFIRMED AND TICKET REISSUED.
NO OPEN RETURNS PERMITTED.
--
IF CHANGES ARE MADE TO OTHER THAN THE FIRST
TICKETED FLIGHT THE FARE LEVELS IN EFFECT AT
TIME OF ORIGINAL TICKET ISSUE WILL APPLY
PROVIDED THE NEW ITINERARY QUALIFIES FOR
ALL THE PROVISIONS OF THE ORIGINALLY
TICKETED FARE FAMILY /
IGNORING ADVANCE RESERVATION OR TICKETING
REQUIREMENTS.
--IF THE NEW ITINERARY QUALIFIES FOR THE
ORIGINALLY TICKETED FARE FAMILY / BUT RESULTS
IN A HIGHER FARE /
THE DIFFERENCE IN FARE AND THE CHANGE FEE
CONTAINED IN THIS RULE MUST BE COLLECTED
AT TIME OF CHANGE/REISSUE.
--IF THE NEW ITINERARY QUALIFIES FOR THE
ORIGINALLY TICKETED FARE FAMILY / BUT RESULTS
IN A LOWER FARE /-- NO REFUND WILL BE MADE--.
-------FULL CHANGE FEE WILL APPLY-----
--
IF CHANGES BY THE PASSENGER NO LONGER QUALIFY
FOR THE ORIGINALLY TICKETED FARE / FULL VALUE
OF THE ORIGINAL TICKET MAY BE APPLIED TOWARDS
EQUAL OR HIGHER QUALIFYING FARE -
USING FARES IN EFFECT ON THE ORIGINAL
TICKETING DATE.
--
FARE IS RECALCULATED FROM POINT OF ORIGIN.
ANY DIFFERENCE IN FARE/TAXES/FEES/SURCHARGE
WILL BE APPLICABLE AT TIME OF CHANGE/REISSUE.
--
B. EMERGENCY PROVISION
--
TICKET MAY BE USED TOWARD THE PURCHASE OF
ANOTHER TICKET DUE TO AN EMERGENCY OF
PASSENGER / IMMEDIATE FAMILY MEMBER OR
TRAVELING COMPANION.
DOCUMENTATION REQUIRED.