Firstly, I'm not Brian (but I'll take it as a compliment!)
Positional data from the Racal SuperTANS, referenced in the AAIB report, indicated that a small overall course change of a few degrees occurred between the waypoint and impact point. There is not enough data to indicate whether the aircraft went from one point to the other in a straight line or was weaving however. What is certainly true is that the aircraft did not make the left turn required to stay on track, or a harder left turn to continue low level up the coast.
This then raises the question as to whether they carried on deliberately, accidentally (because of distraction perhaps) or involuntarily. We can assume that they were aware of the approaching terrain as they entered a climb at some point. This then surely discounts the likelihood of the crew deliberately carrying on, leaving their intended flightpath. It is similarly unlikely (but possible) that both crew members were distracted by something inside the aircraft, such as an engine malfunction indication. In such cases however, procedures existed for dealing with such occurrences where one pilot is heads-in, and there is nothing to indicate that an experienced crew like this would have departed from these procedures. Again, the climb suggests that at least one crew member was aware of the terrain. This only leaves the possibility that they carried on involuntarily.
The assumption (a reasonable one in my opinion also) that the aircraft was capable of maintaining level flight up to the waypoint does not give any proof that it was capable of climbing. We also do not know that a "cruise climb" occurred, all we have is an estimate of height at the waypoint, and the knowledge of the height at which the aircraft crashed. SuperTANS gave no rate of climb data, so we have no actual facts on how the aircraft got from one height to the other.
Fog in the area can be very localised, and there is no evidence that the conditions at the lighthouse extended that far out. You say "how can we say they were not IMC?" - indeed, however by the same token - how can we say they were not? That it cannot be proven that there were visual does not mean that they were not. That it cannot be proved that the aircraft was malfunctioning doesn't prove that it was not.
There was anecdotal evidence that the crew had requested an alternative aircraft (a Mk1) the previous evening, but this request was refused. I think this is in the House of Lords report, but I can't find the reference just now.
This may seem like pedantic wordplay, but it is necessary to be pedantic to ensure that assumptions and estimates do not become accepted as facts. I can't explain the accident using only the absolute known facts, and neither anyone else. Where we differ is in the reliance on assumptions to convict dead men - the majority of us find that unacceptable.