* Johnson: We don't get involved in each other's elections
* U.S. President Trump due to arrive on Dec. 2
* No deal preparations stay in place - Johnson says
* Johnson pitches new state aid rules and 'buy British' (Adds Trump administration official comment)
By Guy Faulconbridge and Andrew MacAskill
LONDON, Nov 29 (Reuters) - Prime Minister Boris Johnson said on Friday it would be "best" if U.S. President Donald Trump does not get involved in Britain's election when he visits London for a NATO summit next week.
Trump waded into the election in October by saying opposition Labour Party leader Jeremy Corbyn would be "so bad" for Britain and that Johnson should agree on a pact with Brexit Party leader Nigel Farage.
On Friday, a senior Trump administration official told reporters that Trump is "absolutely cognizant of not, again, wading into other country's elections."
But senior Conservatives are nervous that Trump could upset the campaign when in London, just over a week before the Dec. 12 election, which polls indicate Johnson is on course to win.
"What we don't do traditionally as loving allies and friends, what we don't do traditionally, is get involved in each other's election campaigns," Johnson, 55, told LBC radio.
"The best (thing) when you have close friends and allies like the U.S. and the UK is for neither side to get involved in each other's election."
Johnson has said that, if he retains power, he will deliver Brexit by Jan. 31 - after nearly four years of political crisis following a 2016 referendum in which Britons voted to leave the European Union.
He said he wanted to keep in place government preparations for a possible no-deal Brexit - under which Britain would leave without agreement on the terms with Brussels and potentially expose itself to more economic uncertainty - but that he expected to secure a trade deal with the EU by the end of 2020.
"Many of those preparations will be extremely valuable as we come out of EU arrangements anyway," he told reporters.
NHS "NOT FOR SALE"
The U.S. president, who is due to arrive on Monday, has cast Johnson as "Britain's Trump," and during a previous visit criticised Johnson's predecessor Theresa May over her Brexit policy.
Labour's Corbyn has said Johnson will sell off parts of Britain's widely cherished health service to U.S. businesses after Brexit, which Johnson has denied. Trump previously said everything including health should be on the table in trade talks, though he later said health would not be.
Johnson said he would use Brexit to introduce new state aid rules, change state purchasing policies and reform farming so that public bodies aim to "buy British" goods.
"The NHS is not for sale," he said.
Taking questions from LBC listeners, Johnson declined to say how many children he had or whether he would have any more.
Johnson, 55, is living at the prime minister's Downing Street residence with his partner Carrie Symonds after separating from his wife last year. (Additional reporting by Michael Holden and Kate Holton, Editing by Andrew MacAskill and Timothy Heritage)