Thank you to MiddleEnglander for collating the information.
There are 5 deferred elections for a total of 13 seats during June along with the first 4 local council by-elections since the General Election. One by-election has already been called in July whilst there are a further 17 known vacancies and impending vacancies, 8 in England, 6 in Scotland and 3 in Wales.
Deferred elections:
4th June
Kettering DC, Rothwell - 3 seats (2 Conservative, 1 Labour in 2011) - 10 candidates: 3 Con, 3 Lab, 3 Green, 1 UKIP
9th June
Wyre Forest DC, Areley Kings & Riverside - 3 seats (boundary change since 2011) - 13 candidates: 3 Con, 3 Lab, 3 UKIP, 3 ICHC, 1 Green
18th June
Christchurch DC, North Highcliffe & Walkford - 2 seats (2 Conservative in 2011) - 6 candidates: 2 Con, 2 Lab, 2 UKIP
Mole Valley DC, Holmwoods - 2 seats (LD in 2011, UKIP in 2014) - 8 candidates: 2 Con, 2 LD, 2 UKIP, 2 Green
25th June
South Kesteven DC, Market & West Deeping - 3 seats (1 Con, 2 Ind in 2011)
By-elections
4th June
Cambridgeshire CC, Wisbech South - UKIP sitting as Independent resigned - 4 candidates: Con, Lab, LD, UKIP
11th June
Sutton LB, Wallington South - LD died - 6 candidates: Con, Lab, LD, UKIP, Green, Ind
Tower Hamlets LD, Stepney Green - Tower Hamlets First disqualified - 8 candidates: Con, Lab, LD, UKIP, Green, 2 Ind, Something New
25th June
Cambridgeshire CC, Romsey - LD resigned
9th July
North Lanarkshire UA, Thorniewood - SNP resigned
Known vacancies in England - 8
East Sussex CC, Old Hastings & Tressell - Labour died 6th May
Hastings BC, Central St Leonards - Labour died 6th May
Hyndburn BC, Spring Hill - Labour died 5th May
Lincolnshire CC, Grantham Barrowby - Conservative resigned around 13th April now MP for Bury St Edmunds
North East Lincolnshire UA, Croft Baker - Labour died 18th May
Northumberland UA, College - Labour died around 15th April
Peterborough UA, West - Conservative resigned around 10th April
Wychavon DC, Droitwich East - Conservative died 19th May
Known vacancies in Scotland - 6
Aberdeen UA, George Street / Harbour - SNP impending resignation
Aberdeen UA, Hilton / Stockethill - SNP resigned around 10th May now MP for Aberdeen North
Aberdeen UA, Kincorth / Loriston - SNP resigned around 10th May now MP for Aberdeen South
Glasgow UA, Calton - SNP resigned around 14th May now MP for Glasgow Central
Glasgow UA, City Centre - SNP resigned around 14th May now MP for West Dunbartonshire
Glasgow UA, Langside - Green resigned due to ill health around 14th May
Known vacancies in Wales - 3
Caerphilly UA, Bedwas, Trethomas & Machen - Labour died 7th May
Caerphilly UA, New Trdegar - Labour announced impending resignation on 14th May now MP for Merthyr Tydfil & Rhymney
Cardiff UA, Pentyrch - Conservative resigned 21st May now MP for Cardiff North