If you were only looking at the flight shown for the outbound half of a round-trip ticket, then there's another factor potentially at work here. The type of fare that you're looking at is priced on a round-trip basis. There isn't any reliable concept of "a price for the outbound flight" with these fares. When you are looking at what ba.com shows you for one direction, it's only an estimate because of the algorithm that divides up into two notional parts all of the possible round-trip prices for all the possible combinations of flights/dates within your search.

In other words, depending on exactly how you search ba.com each day and depending on the availability on all of the other flights within the ambit of each day's search, you could find that one day ba.com is showing you "£300" for "the outbound" and "£200" for "the inbound", and the next day it is showing you "£225" for "the outbound" and "£275" for "the inbound. But actually the underlying fare hasn't changed at all because you're actually being shown a £500 round-trip fare in both cases.

The only reliable price is the specific price quote for a specific pair of flights after you've selected them. But experience on FT shows that many people never both to go this far, because they post about their confusion with the apparent prices being shown for each direction.

This isn't necessarily what's happened to you this time, but it is one of the possible explanations for the phenomenon that people sometimes complain about of prices going up after they've done more searching. If you use all of the available tools to cross-check what ba.com is doing, it is readily apparent that this is not happening.