Will I keep the credits that I redeemed from a Google Play voucher if I get a new phone? I bought a new phone, and I have redeemed a £25 Google Play voucher on my Play Store account. 

I need to take back my phone. If I get a new phone, will I still have the £25 on my Google Play account?
Yes, the credits / load is tied to your account. So, it will still be there, as long as you use the same Google account.