* FTSE 100 up 0.4 pct, mid cap index up 0.1 pct
* Sterling turns back lower after brief spike
* UK formally begins Brexit negotiation process
* 3I rises after upgrade (Adds details, closing prices)
By Kit Rees and Danilo Masoni
LONDON, March 29 (Reuters) - Britain's FTSE index of leading shares closed higher on Wednesday after a choppy session following Britain's formal triggering of its departure from the European Union.
The blue-chip FTSE 100 ended up 0.4 percent, as sterling's retreat stoked expectations more gains for the export-heavy index.
While the June 2016 vote for Brexit spurred a sharp sell-off in equities, the FTSE 100 recovered swiftly and is up more than 16 percent from pre-Brexit levels. It has hit a series of record highs this year.
The benchmark index's predominantly international, dollar-earning firms have been boosted by the weaker pound, which remains down almost 17 percent from pre-Brexit levels. The more domestic-focused mid-cap index rose 0.1 percent.
"It's all about the pound. Any weakness in the pound will see the FTSE 100 push that bit higher," Charles Hanover Investments' partner Dafydd Davies said.
"The banks are a key sector to be keeping a very keen eye on, and of course also any inward-looking UK-listed companies stand to be particularly volatile in relation to the agreements or any guidance that we do get later on in the day."
Though individual stock moves were relatively muted, shares in 3I Group soared 5.7 percent after Morgan Stanley analysts upgraded the private equity firm to "overweight".
"We believe Action could be worth €10bn+ (more than 250p per 3i share). This is not reflected in 3i's share price, so we upgrade to Overweight," analysts said in a note.
EU antitrust regulators blocked a proposed merger of Deutsche Boerse and the London Stock Exchange , saying that the deal would have harmed competition. Shares in LSE rose 2.7 percent.
Mining firms also provided support, with BHP Billiton and Antofagasta gaining 2.7 percent and 2.2 percent respectively as the price of copper hit its highest level for more than a week.
Gold miners Randgold Resources and Fresnillo fell as the price of gold slipped.
Blue chips aside, AA dropped 3.9 percent after brokers chewed over Tuesday's full-year results, while shares in Acacia Mining fell 3 percent after JP Morgan downgraded the gold mining company to "neutral". (Editing by Louise Ireland)