Is yougov.co.uk Safe?

Questions appear to be asked (or dodged) based on prior voting records, resulting in poll results appearing on television that you most certainly weren't asked to take part in and which appear completely at odds with polls taken on-the-street. https://medium.com/@CorbynSuperFan/yougov-messed-with-the-wrong-guy-ecf3abdfb7d2 YouGov claim on their website to have a panel of over 800,000 adults, so why do their surveys always poll around a thousand of them?