London-listed United had option to purchase an additional 10% farmed interest in the licences, although that expired last Thursday (May 31).
Despite this, the company said it remains in talks with its other partners in the licences on taking a further equity stake.
“We have an existing 10% interest in the highly prospective Colter discovery which is due to be drilled later this year,” said UOG chief executive Brian Larkin.
“Although the option has lapsed we remain extremely supportive and positive about its potential and are actively progressing discussions with our partners on increasing our stake in the project.”
The Colter discovery, off England's south coast, is adjacent to Wytch Farm, the largest onshore oil field in Europe, where more than 450mln barrels of oil have been produced.
A previous well at Colter encountered a 10.5 metre oil column and following seismic work in the area the new well will target a location up-dip from the original site.
United, via a farm-in option, has a 10% interest in the project and it estimates that its share of the costs to drill the well will be less than £1mln.
The company describes Colter as an “extremely attractive prospect” and drilling is expected in the second half of the year.
Shares dipped 1% to 5.1p on Monday morning.