All UniprotIDs of a cancer pathway
I would do this through the Reactome database. Searching for "AKT Signalling" returns, among other things, an entry for PI3K/AKT Signaling in Cancer (Homo sapiens) (REACT_147723). Clicking on that link will take you to the pathway's page, and if you click on "Disease" (under "Locations in the PathwayBrowser"), you will be shown a link to the pathway browser page of that particular pathway. 

Once there, click on the "Molecules" tab, select "Proteins" and you will get a list of the proteins involved in the pathway and their UniProt accessions. There, you can click on the "Download" link and retrieve this list:

MoleculeType    Identifier  MoleculeName    
Proteins    P31751  UniProt:P31751 AKT2 
Proteins    P31749  UniProt:P31749 AKT1 
Proteins    Q9Y243  UniProt:Q9Y243 AKT3 
Proteins    P16220  UniProt:P16220 CREB1    
Proteins    Q12778  UniProt:Q12778 FOXO1    
Proteins    P98177  UniProt:P98177-2 FOXO4  
Proteins    O43524  UniProt:O43524 FOXO3    
Proteins    P98177  UniProt:P98177-1 FOXO4  
Proteins    O15530  UniProt:O15530 PDPK1    
Proteins    Q92934  UniProt:Q92934 BAD  
Proteins    P46527  UniProt:P46527 CDKN1B   
Proteins    P38936  UniProt:P38936 CDKN1A   
Proteins    Q00987  UniProt:Q00987 MDM2 
Proteins    P60484  UniProt:P60484 PTEN 
Proteins    Q8WWG1  UniProt:Q8WWG1 NRG4 
Proteins    P10721  UniProt:P10721 KIT  
Proteins    Q9HCT0  UniProt:Q9HCT0 FGF22    
Proteins    P15498  UniProt:P15498 VAV1 
Proteins    P05230  UniProt:P05230 FGF1 
Proteins    Q15303  UniProt:Q15303-1 ERBB4  
Proteins    P08620  UniProt:P08620 FGF4 
Proteins    P42338  UniProt:P42338 PIK3CB   
Proteins    O14944  UniProt:O14944 EREG 
Proteins    O15520  UniProt:O15520 FGF10    
Proteins    P21802  UniProt:P21802-3 FGFR2  
Proteins    Q13480  UniProt:Q13480 GAB1 
Proteins    P55075  UniProt:P55075-1 FGF8   
Proteins    O00459  UniProt:O00459 PIK3R2   
Proteins    Q9UEF7  UniProt:Q9UEF7-1 KL 
Proteins    P09619  UniProt:P09619 PDGFRB   
Proteins    Q8WU20  UniProt:Q8WU20 FRS2 
Proteins    Q86Z14  UniProt:Q86Z14 KLB  
Proteins    O00329  UniProt:O00329 PIK3CD   
Proteins    P11487  UniProt:P11487 FGF3 
Proteins    Q9GZV9  UniProt:Q9GZV9 FGF23    
Proteins    P42336  UniProt:P42336 PIK3CA   
Proteins    P42081  UniProt:P42081 CD86 
Proteins    P35568  UniProt:P35568 IRS1 
Proteins    P04626  UniProt:P04626 ERBB2    
Proteins    Q02297  UniProt:Q02297 NRG1 
Proteins    P12034  UniProt:P12034-1 FGF5   
Proteins    P56975  UniProt:P56975 NRG3 
Proteins    Q9NP95  UniProt:Q9NP95 FGF20    
Proteins    Q02297  UniProt:Q02297-10 NRG1  
Proteins    P22607  UniProt:P22607-2 FGFR3  
Proteins    P21802  UniProt:P21802-1 FGFR2  
Proteins    Q15303  UniProt:Q15303-2 ERBB4  
Proteins    P62993  UniProt:P62993-1 GRB2   
Proteins    O60258  UniProt:O60258-1 FGF17  
Proteins    P11362  UniProt:P11362-1 FGFR1  
Proteins    P15391  UniProt:P15391 CD19 
Proteins    Q9Y4H2  UniProt:Q9Y4H2 IRS2 
Proteins    Q99075  UniProt:Q99075 HBEGF    
Proteins    P21583  UniProt:P21583-1 KITLG  
Proteins    P31371  UniProt:P31371 FGF9 
Proteins    P27986  UniProt:P27986 PIK3R1   
Proteins    P22455  UniProt:P22455 FGFR4    
Proteins    P04085  UniProt:P04085-2 PDGFA  
Proteins    P06241  UniProt:P06241 FYN  
Proteins    P01127  UniProt:P01127 PDGFB    
Proteins    P21802  UniProt:P21802-5 FGFR2  
Proteins    Q06124  UniProt:Q06124 PTPN11   
Proteins    P21781  UniProt:P21781 FGF7 
Proteins    O95750  UniProt:O95750 FGF19    
Proteins    P21860  UniProt:P21860-1 ERBB3  
Proteins    O14511  UniProt:O14511 NRG2 
Proteins    P01133  UniProt:P01133 EGF  
Proteins    O76093  UniProt:O76093 FGF18    
Proteins    P21802  UniProt:P21802-18 FGFR2 
Proteins    O43320  UniProt:O43320 FGF16    
Proteins    P04085  UniProt:P04085-1 PDGFA  
Proteins    P10767  UniProt:P10767 FGF6 
Proteins    P06239  UniProt:P06239 LCK  
Proteins    Q92569  UniProt:Q92569 PIK3R3   
Proteins    Q6PIZ9  UniProt:Q6PIZ9 TRAT1    
Proteins    Q9UEF7  UniProt:Q9UEF7-2 KL 
Proteins    P35070  UniProt:P35070 BTC  
Proteins    P16234  UniProt:P16234 PDGFRA   
Proteins    P33681  UniProt:P33681 CD80 
Proteins    P22607  UniProt:P22607-1 FGFR3  
Proteins    P09038  UniProt:P09038 FGF2 
Proteins    P10747  UniProt:P10747 CD28 
Proteins    P00533  UniProt:P00533 EGFR 
Proteins    P55211  UniProt:P55211 CASP9    
Proteins    P22736  UniProt:P22736 NR4A1    
Proteins    Q6ZVD8  UniProt:Q6ZVD8 PHLPP2   
Proteins    O60346  UniProt:O60346 PHLPP1   
Proteins    Q9UKV8  UniProt:Q9UKV8 AGO2 
Proteins    Q9HCE1  UniProt:Q9HCE1 MOV10    
Proteins    Q9UPQ9  UniProt:Q9UPQ9 TNRC6B   
Proteins    Q9HCJ0  UniProt:Q9HCJ0 TNRC6C   
Proteins    Q9UL18  UniProt:Q9UL18 AGO1 
Proteins    Q8NDV7  UniProt:Q8NDV7 TNRC6A   
Proteins    Q9H9G7  UniProt:Q9H9G7 AGO3 
Proteins    Q9HCK5  UniProt:Q9HCK5 AGO4 
Proteins    Q96RU7  UniProt:Q96RU7 TRIB3    
Proteins    Q5T1C6  UniProt:Q5T1C6 THEM4    
Proteins    Q96B36  UniProt:Q96B36 AKT1S1   
Proteins    P49841  UniProt:P49841 GSK3B    
Proteins    P49840  UniProt:P49840 GSK3A    
Proteins    O15111  UniProt:O15111 CHUK 
Proteins    Q9BPZ7  UniProt:Q9BPZ7 MAPKAP1  
Proteins    Q6R327  UniProt:Q6R327 RICTOR   
Proteins    Q9BVC4  UniProt:Q9BVC4 MLST8    
Proteins    P42345  UniProt:P42345 MTOR 
Proteins    Q9UBS0  UniProt:Q9UBS0 RPS6KB2  
Proteins    P49815  UniProt:P49815 TSC2