Best way to get/spend money while visiting the UK?

Since moving to the UK I've used my US debit card here all the time. My bank still has my old address in the US (my parents) and I never notified them that I had moved but have had no problems using it here. I would take a few £'s with you just incase of card probs once you arrived. A lot of places locally to me don't take cheques of any kind including travellers cheques so you might want to consider that before you purchase them.

This can be a very, very bad move from a UK tax perspective bcause each remittance (ie use of US bank card to bring money over) could include a currency gain which may be subject to UK capital gains tax at the 18% rate.  Sadly, it will also make preparing UK tax returns extremely complicated.

Is that subject to the annual exempt amount?  And if it is, is the exempt amount based upon what is withdrawn or gains from the currency exchange.  If it's gains from the exchange, how do you determine how much you've gained?

If you claim the remittance basis there is no annual exemption at all so you might choose not to claim the remittance basis and pay UK tax on worldwide income...

Is there any difference, exchange rate-wise, between bringing actual cash dollars to convert into pounds as needed, vs. making cash withdrawals in the UK using my US ATM card?

I found that I got a better exchange rate from the ATM, but I was a Bank of America customer so never paid any fees for withdrawals in the UK (using Barclays).  If you're going to be paying ATM fees every time you withdraw, though, it might cost about the same as exchanging cash in the end.

Is there any difference, exchange rate-wise, between bringing actual cash dollars to convert into pounds as needed, vs. making cash withdrawals in the UK using my US ATM card?

I've never done a side-by-side comparison, but my understanding has always been that most  money exchange centers (Travelex, etc) have poor exchange rates in comparison to ATMs; in addition, they will charge you enormous fees.  The post office may be much better, but then you have to find a post office.  I don't remember seeing post offices in any of the airports I've flown into--they may very well be there, but they aren't around every corner like ATMs. You should probably call your bank and ask them what kinds of overseas ATM fees and charges they have, as well as what their exchange rate is based on.  The fees from my bank have always been really low and the exchange rate really good, but I imagine this varies.

For what it's worth, I always use ATMs with a small amount of cash to account for possible ATM failures.

Keep in mind not all ATMS will work or at least for some banks not all ATMs in the UK will work.   I can use any bank ATM but the privates never work for me, I assume it has something to do with my bank.

I went to a post office once and was kind of shocked at the amount I got back (even with understanding what the exchange rate was), from then on I have always used the ATM but I never have to worry about ATM costs because my bank refunds all of them. 

Good luck!