Upgrades to Carrera Crixus
Just buy the best bike you can get for the money you have. Component upgrades cost less money when bought as part of a whole bike, compared to being purchased and fitted separately.

As an example, the price difference between a bike fitted with 105 and one with Tiagra is a lot less than what a 105 groupset costs.

The difference can be defrayed by selling the original components as new take-offs, but it's a hassle and you probably still will not make up the difference.

If I were you I'd look for a used bike in good shape, rather than deal with Halfords level bikes.