Generally london (if you are coming up from the OTR/UMBEL direction) will transfer you to Scottish generally 126.925 (Montrose sector) at your cruising level.
The FL260L by NATEB is a standing agreement for the next sector (124.5 - Tay sector). This restriction applies to all Scottish TMA inbounds. This is designed to prevent penetration of the DCS sector. Where traffic and controller workload permits often the NATEB restriction is ignored and then we will go straight to the FL220L OTBUN restriction with co-ordination of the adjacent sector.
You are correct that there is a danger area below L602. However this is D512 (otterburn range). Spadeadam is D510 which is to the SW of L602. The base of L602 is FL205. Therefore you will often be given the next clearance to decend to FL210 on cantact with 126.3 (Talla sector).
It is rare that D512 is active above 12000ft but the controller will usually step you down on the bases of controlled airspace. Any decent further that this will take you outside controlled airspace.
Even if D512 is not active, it is often that military targets are transiting the area enspecially when Spadeadam is in use. Therefore controllers will generally keep the aircraft inside controlled airspace for your protection and ours
The Talla controllers all understand that the L602 base levels keep most aircraft well above the ideal decent profile. If it suits you could always ask for decent outside controlled airspace and request the appropriate service, however dont be suprised if this is denied due to unknown aircraft outside controlled airspace and controller workload.
As for direct routings, if the frequency appears to be busy, or you can hear what ytou think could be a complex or unusual situation then dont even ask. However if it seems quiet then by all means, we are all happy to help.
Hope this helps