Remortgage advice - adverse credit

Hi, we are looking for advice in re-mortgaging our property, to release equity for home improvements. The current mortgage is with NRAM on their standard variable rate (4.54%). The outstanding balance is ~£165000 from an initial £200000 mortgage. The value of the property in its current condition is £230000 to £250000. A property in comparable condition sold for £240000 in April. We know that our valuation could be improved to ~ £300,000) through renovation.
To release equity, a change of mortgage provider will be required. There are a number of additional factors to consider; a below average credit score, including mortgage defaults dating back to 2012. Those arrears were cleared in full in December 2012 and since that date all payments have been made in full and on time. Additionally there is a manageable level of historic household credit card debt with interest free arrangements with numerous providers and current low interest credit card debts.
Can anyone tell us about products that would be suitable for us or other options that we have not considered? Thank you.