I think the confusion here is, in the UK, the 250/FL100 limit does not apply unconditionally (unlike e.g. in the US).
§ 91.117 Aircraft speed.
(a) Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).
Airspace Speed Limit (SERA.6001)
Aircraft flying below FL100 are required to observe, with exceptions, a speed limit of 250 kt IAS. Such a limit is an essential component of the ‘see and avoid’ principle when separation is not established by ATC. This is in addition to speed limits, which may be notified for specific procedures.
The 250 kt speed limit does not apply to:
1. (1) flights in Class A and B airspace;
2. (2) IFR flights in Class C airspace;
Between SSE11 and CLN, you are actually in class A airspace and technically exempted from the 250 speed limit.
However, most SID/STAR procedures near major UK airports do (re-)introduce the restriction, which is why all corresponding charts (be they from Jeppesen or the AIP, though the former do highlight it in red) specifically mention the speed limit (because it’s actually mandated by the procedure itself rather than the general rule).
So, in theory, after SSE11, you are allowed to accelerate past 250 knots, but in fact you aren’t because the procedure says otherwise (distinct limitation).
What makes it very confusing is, almost all SID and STAR procedures in the UK mandate a 250/FL100 restriction, so you’d think it’s a general rule like in the US, but no – it’s actually implemented on a per-procedure basis.
That’s probably why Jeppesen coded the procedure this way (Navigraph only translates their coding to the appropriate addon format).
And yes, once you are cleared above FL100, it does mean you also need to manually clear the speed in your FMS, which is cumbersome and/or counter-intuitive, but that’s actually a result UK’s somewhat unusual regulations…