An Ancestry study has this to say (slightly reformatted for readability):
This is according to new analysis of the genetic history of two
million people worldwide by Ancestry, the leader in family history and
consumer genomics, based on data collated from the AncestryDNA home
DNA test that examines a person’s entire genome at over 700,000
different genetic locations.
The results reveal the genetic ethnic make up of the ‘average’ person
in the UK and what countries and/or regions they can trace their
ancestry back to over the past 500 years. They found that the average
UK resident is
- 36.94% British (Anglo Saxon),
- 21.59% Irish (Celtic) and
- 19.91% Western European (the region covered today by France and Germany).
Following these top three regional ethnicities in the average UK
- Scandinavia (9.20%), the
- Iberian Peninsula> (Spain/Portugal) (3.05%),
- Italy and Greece (1.98%).
Note that these figures are an average across the results. There is more concerning variation by region within the UK:
English people have significantly less Irish ancestry (just 20% of
their genetic make-up) on average compared to people living in
Scotland (43.84%), Wales (31.99%) and Northern Ireland (48.49%).
English residents do however have the highest amount of Scandinavian
(9.39%) and Western European (French/German) (20.45%) ancestry.
Scottish residents have the highest amount of Finnish/Northwest
Russian (1.31%) heritage, which is explained by their geographic
Welsh residents have the highest proportion of ancestry
from the Iberian Peninsula (Spain/Portugal) in the UK (3%).
This type of family history DNA study has little to do with ancient Celts, which is apparently the thought the OP has. Note the bounds mentioned at the beginning of the cited article (emphasis mine):
Study looked at the nation’s ethnicity dating back 500 years from 26
These 'ethic' DNA studies do not have enough data to go back to the ancient Celts, Gauls or Romans. They just indicate you have DNA similar to someone whose family has lived in one of these locations for a long time (the last 500 years).