It seems to me that the package rworldmapprovides some additional features on top of the package maps.
If this is true, you can do the following:
library(maps)
x <- map("world", plot=FALSE)
str(x)
List of 4
$ x : num [1:27121] -130 -130 -132 -132 -132 ...
$ y : num [1:27121] 55.9 56.1 56.7 57 57.2 ...
$ range: num [1:4] -180 190.3 -85.4 83.6
$ names: chr [1:2284] "Canada" "South Africa" "Denmark" "Great Lakes:Superior, Huron, Michigan" ...
- attr(*, "class")= chr "map"
maps, names . , . , , :
x$names[grep("UK", x$names)]
[1] "UK:Gibralter"
[2] "UK:Scotland:Isle of Lewis"
[3] "UK:Pitcairn Island"
[4] "UK:Guernsey"
[5] "UK:Great Britain"
[6] "UK:Scotland:Shetland Islands:Unst"
[7] "UK:Saint Mary's"
[8] "UK:Scotland:Shetland Islands:Yell"
[9] "UK:Northern Ireland"
[10] "UK:Bermuda"
[11] "UK:Tristan da Cunha Island"
[12] "UK:Scotland:Saint Kilda"
[13] "UK:Scotland:Ruhm"
[14] "UK:Scotland:Benbecula"
[15] "UK:Scotland:North Uist"
[16] "UK:Saint Helena Island"
[17] "UK:Scotland:Island of Skye"
[18] "UK:Scotland:Barra"
[19] "UK:Scotland:Island of Mull"
[20] "UK:Henderson Island"
[21] "UK:Isle of Sheppey"
[22] "UK:Jersey"
[23] "UK:Scotland:Coll"
[24] "UK:Scotland:Jura"
[25] "UK:Scotland:Island of Arran"
[26] "UK:Scotland:Tiree"
[27] "UK:Scotland:Islay"
[28] "UK:Ascension Island"
[29] "UK:Scotland:Colonsay"
[30] "UK:Scotland:Shetland Islands:Mainland"
[31] "UK:Scotland:South Uist"
[32] "UK:Scotland:Orkney Islands:Hoy"
[33] "UK:Gough Island"
[34] "UK:Scotland:Orkney Islands:Mainland"