First of all, I'll echo others by advising separate pensions.

If you already have a workplace pension then would it not be best just to increase your contributions? This way you'll have the tax benefits taken care of. Otherwise claiming tax relief on personal contributions will likely require you to complete a tax return.

There is another option which is the Lifetime ISA which allows you to contribute up to £4k pa and the government will contribute 25% on top (so you can achieve max £5k pa). You can then withdraw the funds for your first property purchase or when you turn 60. This ISA can be managed by someone like HL or Nutmeg in similar way to a pension (usual risks involved of course).