If it helps, when I went back to Barclays near the embassy they could only provide me with the six months statements printed from the ATM (that actually have a bank stamp on then) .. I suggest perhaps you try the same with HSBC so you have a set all identical showing the last 6months transactions including the transfer from ISA to standard account..
i submitted my application in person, they informed me that both thai bank and ISA statements were not acceptable. I then found the local Barclays and requested 6 months of my main current account. I them returned and resubmitted.
no reason was given.. but they were very clear in that it had to be in my main/current account linked to my ATM card and overdraft facility. the ISA and secondary bank account I use for rental income was not acceptable.
I learnt this the hard way.. Strangely funds where in a instant cash ISA (so immediately available). What they accepted was exactly the same account details just with the funds in a section linked to my ATM card and overdraft facility..
I got a METV from London end of last year. first I showed 6 months of funds In a thai bank account, this was unacceptable had to be an account in my home country.. Then I showed 6 months in a UK Barclays account funds held in cash ISA, and this was also unacceptable. So I then had to get 6 months statement showing funds in a current account with linked ATM card... finally they accepted!
I have recently done the same crossing on a Multiple entry tourist visa issued in the UK. I had no problems at all although they did want to see the 20k cash..l