welcome garveinho

others have given some good advice, but perhaps you are confused on TP (tier points) vs miles.

Miles = broadly corresponds to the distance flown. Can be purchased (up to 20.000 per year IIRC). Can be used to buy flights, or upgrade tickets...subject to availability and fare rules of course! Miles expire if there is no activity in your BA account for 36 months

Tier Points = the way of BA keeping score of how much you flew in a given 12 month period, and assigning you to Blue, Silver, Gold level. Tier Points are fixed according to Short Haul (broadly, Europe) and Long Haul. You get more TP for flying in a premium class (WT+, Club, First).

TP are reset at the end of your membership year (and when first reaching Silver or Gold). You can't directly purchase TP without flying.

Now you hopefully have enough background to enjoy your read of the BA Dashboard ...I warn you, you'll come out of there with lots of ideas for flying