Statement Of Changes In Equity [Abstract]
| Concept |
As at 2021-12-31 |
2021-01-01 to 2021-12-31 |
2021-01-01 to 2021-12-31 |
2021-01-01 to 2021-12-31 |
2021-01-01 to 2021-12-31 |
2021-01-01 to 2021-12-31 |
2020-01-01 to 2020-12-31 |
As at 2020-12-31 |
2020-01-01 to 2020-12-31 |
2020-01-01 to 2020-12-31 |
As at 2019-12-31 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Statement of changes in equity [abstract] | |||||||||||
| Statement of changes in equity [line items] | |||||||||||
| Equity |
1,330,700,000
GBP
|
— | — | — | — | — | — |
1,270,600,000
GBP
|
— | — |
1,202,400,000
GBP
|
| Changes in equity [abstract] | |||||||||||
| Comprehensive income [abstract] | |||||||||||
| Profit (loss) | — | — | — | — |
119,500,000
GBP
|
119,500,000
GBP
|
77,400,000
GBP
|
— |
77,400,000
GBP
|
— | — |
| Other comprehensive income | — | — |
28,300,000
GBP
|
— |
28,300,000
GBP
|
— | — | — |
21,100,000
GBP
|
21,100,000
GBP
|
— |
| Comprehensive income | — |
–
GBP
|
— |
–
GBP
|
91,200,000
GBP
|
— | — | — |
98,500,000
GBP
|
— | — |
| Dividends recognised as distributions to owners | — |
–
GBP
|
–
GBP
|
–
GBP
|
30,800,000
GBP
|
30,800,000
GBP
|
30,100,000
GBP
|
— |
30,100,000
GBP
|
–
GBP
|
— |
| Increase (decrease) through share-based payment transactions, equity | — |
–
GBP
|
300,000
GBP
|
–
GBP
|
300,000
GBP
|
–
GBP
|
–
GBP
|
— |
200,000
GBP
|
200,000
GBP
|
— |
| Increase (decrease) in equity | — |
–
GBP
|
28,600,000
GBP
|
–
GBP
|
60,100,000
GBP
|
88,700,000
GBP
|
47,300,000
GBP
|
— |
68,200,000
GBP
|
20,900,000
GBP
|
— |