Catch 22, You need a valuation to get a mortgage, but you need a mortgage agreed before you bid, I am afraid.
Otherwise it could be a gamble on being in breach of contract, if you leave it all until after you have won the property at auction, were they to refuse you a mortgage after you had bid for it.
I bid and won a property at auction like you are planning which needed renovating, but I was lucky in that my previous property had been sold and was sat in the bank.
You can get good bargains at auction as most of the prospective buyers are speculators or investors, if you are buying it for private use you can usually go above the prices they are willing to pay. I paid 25% less than it's rough true market value at the time, this saving allowed me to then renovate, which then pushed the value up by around 40%.