Brexit party is indeed different in structure & financing 

all other UK political parties allow membership and
- at least to some extent - votes for delegates to indicate what policies they want
Farage only allows supporters, who can only donate;
they cannot get in his way by having different opinions to him
The Brexit party has NO members - except maybe Farage
his use of Paypal for donations reduces the ability to trace who is paying for it all
and whether foreign donors are breaking the £500 limit, maybe with automatically generated multiple smaller sums
Farage's fundraising for Brexit Party allows foreign donors to 'break rules', warns elections watchdog
https://www.independent.co.uk/news/uk/politics/brexit-party-donations-farage-electoral-commission-paypal-eu-elections-a8925276.html
Nigel Farage’s use of PayPall* to raise huge numbers of small donations for his Brexit Partyy* allows foreign donors to “evade the rules” a watchdog has warned.
The Electoral Commission said its investigation of the party’s fundraising methods is “ongoing”, after visiting its headquarters
.....
“This risk is that it increases the potential for individuals or organisations to evade the permissibility rules,
which primarily seek to prevent significant sums entering UK politics from overseas,”
it explained.
The commission stressed that it was the Brexit Party’s “responsibility” to “ensure it has the systems in place to maintain its compliance with the law”.
“Our review of the systems in operation by the Brexit Party is ongoing,”
....
the Brexit Party’s chairman admitted he did not know whether its PayPal account was taking foreign cash, saying:
“I don’t know what currencies people are paying in.”
< or obviously where they live / their nationality >
...
The PayPal method seeks sums of less than £500 because they are not recognised as official donations – which means the giver does not need to be identified.
Only gifts of £500 or more must be given by a “permissible donor”, who should either be somebody listed on the UK electoral roll or a UK-operating business registered at Companies Housee^.
However, parties are expected to use methods that guard against foreign donors making multiple small donations that, in total, exceed the £500 limit.