Free entry doesn't feel appropriate for somewhere like BPB (many reasons for this) but making everyone pay full price for an attraction where non-riders don't really have any benefits can be a real turn-off for visitors, as mentioned above. With other parks like Towers, Chessy, Legoland, Paultons (not so much Thorpe) - there are a range of things for non-riders to access that make paying to enter feel better value than just walking around a densely concrete & steel populated area.
Something that could work as a better pricing model for BPB would be, as some have already mentioned, a full price rider option & a cheaper non-rider option (both subject to dynamic pricing for peak/off peak times). A non-rider entry could include a set amount of game tokens, a food/drink voucher, money off the shop for example... offering better value to the non-rider and also, promoting footfall to games, retail and food outlets that people may not have had a budget for otherwise, especially on quieter days. There is of course, an issue of non-riders who would have happily paid full price just to come with friends and family now having a cheaper option and getting something from it in the process but, it's important to get people through the door - groups in particular.