In gerneral the separation standard in the NAT HLA will be:

• Lateral: 60 NM
• Vertical: 1000 ft

With Performance Based Communications and Surveillance (PBCS), the lateral separation can be reduced to 23 NM, but there is no mention of this in this older (2017) question.

ICAO Doc 7030 – NAT Regional Supplementary Procedures (SUPPS)
6.2 SEPARATION
6.2.1 Lateral

6.2.1.1 Minimum lateral separation shall be:

a) 110 km (60 NM) between aircraft which meet the minimum navigation performance specifications (MNPS) provided that a portion of the route of the aircraft is within, above, or below MNPS airspace;
b) 167 km (90 NM) between aircraft operating outside the MNPS airspace and at least one aircraft does not meet the MNPS.

1) Between the Iberian Peninsula and the Azores Islands; and

2) Between Iceland and points in Scandinavia and in the United Kingdom;

c) 167 km (90 NM) between aircraft operating outside MNPS airspace where no portion of the route of the aircraft is within, above, or below MNPS airspace:

1) between the United States/Canada and Bermuda; and

2) west of 55°W between the United States, Canada or Bermuda and points in the CAR Region;

d) 223 km (120 NM) between other aircraft; except that lower minima in 5.4.1.2 of the PANS-ATM may be applied, or further reduced in accordance with 5.11 of the PANS-ATM, when the conditions specified in the relevant PANS-ATM Provisions are met.

6.2.1.2 In the practical application of the minima in 6.2.1.1 a), b), c) and d), tracks may be spaced with reference to their difference in latitude, using one degree instead of 110 km (60 NM); one and one-half degrees instead of 167 km (90 NM); and two degrees instead of 223 km (120 NM), provided that in any interval of ten degrees of longitude, the change in latitude of at least one of the tracks does not exceed:

a) three degrees at or south of 58°N;
b) two degreed north of 58°N and south of 70°N; and
c) one degree at or north of 70°N and south of 80°N.

At or north of 80°N, or where the above rates of change of latitude are exceeded, the required lateral separation must be ensured by reference to the track spacing expressed in nautical miles.