Yes, you should have no problem upgrading to a better CPU or GPU later in the future.

The only limitation could be the wattage of the PSU if you wanted to upgrade the GPU, you could get the 500W or 600W version to be safe.

CPU upgrade no problem. GPU upgrade, not so much. At this point, your next best option over a 970 is Fury, or Fury X/980Ti, which a 400W unit would be too tight for. You could upgrade to an R9 Nano with that PSU though, if you wanted. However, by the time you need an upgrade, newer gen GPUs will be out, and you may be able to get a logical upgrade that will perform fine with that PSU. However, the PSU is the first upgrade I'd be making with that rig, to something higher wattage, higher rated probably, and semi/fully modular.

Good rig though, best performance you'll get for that budget. If you wanted a more rounded rig, with better upgrade potential, you'd need to sacrifice GPU performance.

Sanctorum wrote CPU upgrade no problem. GPU upgrade, not so much. At this point, your next best option over a 970 is Fury, or Fury X/980Ti, which a 400W unit would be too tight for. You could upgrade to an R9 Nano with that PSU though, if you wanted. However, by the time you need an upgrade, newer gen GPUs will be out, and you may be able to get a logical upgrade that will perform fine with that PSU. However, the PSU is the first upgrade I'd be making with that rig, to something higher wattage, higher rated probably, and semi/fully modular.

Good rig though, best performance you'll get for that budget. If you wanted a more rounded rig, with better upgrade potential, you'd need to sacrifice GPU performance.

Would it be a smart idea to buy a bigger psu before i buy this build?

Up to you. There are a couple of decent options for not a huge premium in cost. If you can spend a little extra for a slightly better PSU, it could be worth it.

You could get the 500/600W model of the same PSU;
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or a gold rated PSU;
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or if you want something a little tidier;
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Gold rated, and semi-modular so cable management is much easier, great quality unit at a good price. Though, again wattage is the limiting factor here. You'll be fine to upgrade to an NVidia card, but higher end AMD cards tend to use more power, and would be a little tight with a 450W unit. 500/550W would be more than enough for any single GPU rig, but you're looking at an extra £10 over the Seasonic unit for a similar quality 550W unit.