I do not know whether the WGC 'starter' would be 'double blocked' as there is no way to see the signal in the rear from the platform, but it is on a straight and level section, so could probably be 'read through' further back than the previous signal from a point just south of Digswell (i.e. Welwyn) Viaduct and where the up slow diverges. The signal is a controlled signal, because it protects the junction ahead where up trains from platforms 3 and 4 join the up slow from the flyover (these are mostly trains to Moorgate or the peak slow service to King's Cross) and controls movements across the crossover after WGC from the up slow to the up fast. Thus it is sometimes possible to see this signal red, while the signal in advance (which can be seen from the platform) is green.
All the trains involved had previously been held up approaching Stevenage. This is because until shortly before they passed trains were being diverted via the Herford Loop and this is only accessible through the up slow platform (one) at Stevenage. 1A06 was first after the block was lifted to be allowed through the fast line platform (two) at Stevenage and take the normal route through WGC (albeit only on the up slow line through WGC and Hatfield) and so had a clear road ahead, so approached fast and stayed fast. 1T11 was on the slow line at Stevenage, appears to have been checked at Woolmer Green junction (from RTT times). This means that initially it would have been on restrictive signals but these would rapidly cleared as 1A06 (already at speed) covered more distance while it made a standing start.
It is possible that the member of staff on the platform at WGC and who observed 1A06 pass and did shout a warning to those nearby, requested that the two trains after 1A06 be cautioned, but they would have had little time to have done this between the passage of 1A06 and 1T11 and for control to have relayed the request to the driver of 1T11. Thus I am as convinced as I can be from the evidence that the driver of at least 1T11 had chosen to pass WGC at a lower speed (even if that of 1Y05 was cautioned). The speed 1T11 and 1Y05 passed was consistent with the speed I have seen occasional similar moves in the past, whereas the speed of 1A06 was something I had not previously seen through the up slow platform in 11 years of commuting and made enough of an impression to motivate me to start this thread!