Old British Calendar?

Can anyone shed light on this please?

BRT:2:6 - Jesus our Master Light of our Life was hung on the shameful cross in his 27th year this being the one thousand and ninety ninth year of Britain in the reign of Tiberius ruler of the Roman lands to the east. -

(Now Tiberius reigned 47BC - 37AD so how does that sit with it being 1099?)

It's referring to the year count that the Britons had back then
OK do you know when that year count started?
Well not really but if you know that Tiberius ruled during 47BC then substract 1099 from it and you'd have the time frame the count started.
Yes I can work it out :)