Check before you spend
Before planning upgrades, check whether the property is still within its warranty period, because altering installed elements can affect cover on those parts. Also check the extractor performance and the silicone junctions - developer-standard sealing is the most common early weak point.
The upgrades that change daily experience
In order of impact for the money: a better thermostatic shower valve and head, a proper heated towel rail position, improved lighting with a shaver-safe mirror light, and replacing the builder's basin taps. None of these require re-tiling.
Upgrades that need more work
Replacing a small builder's basin with a vanity unit gives storage but may need waste and supply repositioning. Swapping a shower-over-bath for a walk-in shower is a full conversion. Re-tiling to a larger format usually means removing the existing tiles rather than overlaying.
Keeping the layout
New-build bathrooms are laid out around the soil-pipe position, which is rarely worth moving in a flat or a timber-frame house. Working within the existing layout typically halves the cost of an upgrade and avoids the noisiest, most disruptive part of the work.