* Total value of UK M&A reaches $466.5 billion in 2018
* Bankers expect fewer $10bn-plus mega-deals in 2019
* Brexit among geopolitical risks weighing on sentiment
* British firms could be vulnerable if Brexit sparks sell-off
By Ben Martin
LONDON, Dec 21 (Reuters) - Bankers are bracing for a drop in mega-deals in Britain in 2019 as companies cautious about Brexit and other geopolitical risks row back on big transactions following the best year for UK mergers and acquisitions in three years.
Advisors caution that the bitter trade war between the United States and China, a no-deal Brexit and tighter regulation could all deter companies from pursuing more ambitious acquisitions.
"Our view is that next year will be a decent year for M&A but there will be less of the mega-deals," said Eamon Brabazon, the co-head of EMEA M&A at Bank of America Merrill Lynch.
"Given the potential direction of geopolitics, the world's a bit more cautious and so companies may be less likely to do a $10bn-plus deal in 2019," he added.
The value of deals involving British companies climbed to $466.5 billion in the last 12 months, up from $362.7 billion in 2017 and the highest since 2015, according to Refinitiv data.
Outbound M&A by UK firms accounted for $179.6 billion of the volume this year, while inbound deals totalled $142.2 billion and domestic deals between British businesses reached $87.3 billion.
The value of M&A in the UK - which was the world's third largest market for deals - rose even as the number of transactions slid by 7.7 percent to 4,568, as firms pursued fewer but bigger acquisitions.
Globally there were 42 mega-deals worth more than $10 billion, the most since 2015, the Refinitiv data show. Those involving British companies included Vodafone's $21.8 billion acquisition of Liberty Global's assets in Germany and eastern Europe and Comcast's $40 billion purchase of Sky.
Fears that Britain could crash out of the EU without a deal have intensified since November, when Prime Minister Theresa May struck a withdrawal agreement with Brussels that has since met with fierce opposition from UK lawmakers.
Brexit concerns have hit the UK M&A market in recent weeks and were blamed for the collapse last month of a potential 2.9 billion-pound takeover of British shopping centre owner Intu Properties by a consortium including Canadian property giant Brookfield.
Even so, some bankers believe the UK's looming departure from the EU could spur bids for British companies if there are sharp sell-offs in sterling and equities that make London-listed firms look particularly cheap.