This was a very mild December with a CET of 7.3, not as mild December 1988 (7.5) but up to the 19th, it was exceptionally mild as can be seen with the CET trackometer

December 1898
1.  7.9
2.  9.3
3.  9.4
4.  9.8
5.  10.4
6.  10.7
7. 10.2
8.  9.8
9.  9.5
10. 9.5
11. 9.7
12. 9.8
13. 9.4
14. 9.3
15. 9.2
16. 9.0
17. 9.0
18. 9.2
19. 9.0 cf. (8.8 in 1934, 7.5 in 1974, 7.9 in 1979 8.2 in 1985, 6.8 in 1988; 7.5 in 1994; 7.9 in 2006)
20. 8.7
21. 8.4
22. 8.2
23. 7.9
24. 7.8
25. 7.7
26. 7.7
27. 7.8
28. 7.7
29. 7.6
30. 7.5
31. 7.3

Tropical maritime air flooded the UK frequently during that first half of the month.

Christmas Day 1898 has the classic Bartlett set up

Daily reports
North Cadbury: Exceedlingly warm with high minima and excessive humidity. For nearly 60 hrs from 4th to 6th, the temp varied from 57.0F to 51.5F

Wakefield: The shade temp was continuously above 50F from noon on 3rd to 3pm on 6th, the max. on 5th being 59.5F

Monmouth: The min in shade was above 50F each day, that of 6th being 54.9F (4th-6th)

Eastgate: Min temp. 53F (5th)

Kensington: Min temp  53F (5th), 53.6F (6th)

Monthly reports
Kensington: Remarkably warm, 7.2F above average. On 21 days, temp exceeded 50F and on 6 nights did not fall below 50F

Birchington: Extremely mild, the shade max, being above 50F on 20 days, while the min. was above 50F on the 5th and 6th.

Bournemouth. Temp above 50F on 20 days and the min on 6th was 53.3F. Frost on one night</p><p>St Albans. Mean temp 6.3 above the average.

Stamford: Week ending the 10th, mean temp was 48.3F

Tavistock: Temp rose above 50F on 17 days

North Cadbury: Mean temp above 50F on 20 days and never below 40F. Mean max: 52.2F

Cheltenham: Mean temp. 46.1F, ax temp above 50F on 21 days.

Ross-on-Wye: Mean temp 46.4F, 19 days above 50F

Bolton: Mean temp 45.7F, only one frost

Goldsborough Hall: max temp of 57F on 5th

Colwyn Bay: Mean temp 48.0F

Tenterden: Remarkably warm for the first 19 days. Mean temp 45.0F

Wintney: Christmas rose, violets and wallflowers in bloom on 18th

Winslow: On 18 days, the max ranged from 50f to 56f

Norwich: Extraordinarily mild, the mean temp for the 18 days being 47.5F
Max on 5th: 55.6F, min: 53.6F
Max on 6th: 55.2F, min: 52.4F

Torquay: mean temp of 48.8F

Haverfordwest: grass min on 25th of 51.6F

Cargen: Mean temp of 44.5F, pastures green, snowdrops in bloom</p><p>Edinburgh: Mean temp of 44.0F, max temp of 57.5F on the 5th and a mean of 54.0F

Waterford: Fences golden with furze bloom.</p><p>Dublin: Mean temp 6.3F above average. Scarcely a trace of frost, snow, sleet or hail. SW or W prevailed throughout.</p><p> </p>

This is quite amazing seeing this as it implies that a December CET of 9C or even higher is perhaps possible.

Until I saw these CET daily figures I often thought that December 1974 was as mild as possible one could get for December - It seems even higher than 8C is possible.

Just out of curiosity, I wonder what the rest of that winter was like? Did January and February 1890 continue the tropical-maritime-type regime?

January and February 1899 were largely mild but not as mild as December. Any cold spells were short lived.