Yes, 0800 numbers are free to call on Tesco Mobile phones in the UK. Reason being, since 1st June 2015 the law changed in the UK to ensure that all 0800 and 0808 numbers must be free of charge not only from landlines but from mobile phones too.
Prior to June 2015 most mobile phones charged you for calling traditional freephone numbers but Ofcom has now caught up with the times (a generation where mobiles are more popular than landlines) by establishing telecoms regulation reforms to ensure that freephone numbers are now truly free on all popular devices. So whether you’re using a Tesco mobile or a mobile on another network, or even a landline, you can rest assured that when you call a freephone (0800 or 0808) number in the UK you won’t be charged a penny and the call won’t eat into your free minutes allowance either.